Merge #151: Refactor user_output.rs into folder module structure

3df3405 fix: resolve clippy linting warnings in user_output module (copilot-swe-agent[bot])
93e8f27 style: apply rustfmt to user_output module (copilot-swe-agent[bot])
d046477 refactor: convert user_output.rs to folder module structure (copilot-swe-agent[bot])
eaa57be Initial plan (copilot-swe-agent[bot])

Pull request description:

  Refactor user_output.rs Module to Folder Structure

  This PR refactors the monolithic `src/presentation/user_output.rs` file (4,226 lines) into a well-organized folder module structure following project conventions.

  ## Changes

  ### Module Structure
  ```
  user_output/
  ├── mod.rs              # Public API re-exports
  ├── core.rs             # UserOutput implementation + tests (2533 lines)
  ├── theme.rs            # Theme configuration + tests
  ├── verbosity.rs        # VerbosityLevel + VerbosityFilter + tests
  ├── channel.rs          # Channel enum
  ├── traits.rs           # OutputMessage, FormatterOverride, OutputSink
  ├── messages/           # 7 message type implementations
  │   ├── progress.rs
  │   ├── success.rs
  │   ├── warning.rs
  │   ├── error.rs
  │   ├── result.rs
  │   ├── steps.rs        # with builder
  │   └── info_block.rs   # with builder
  ├── sinks/              # 4 sink implementations
  │   ├── writers.rs      # StdoutWriter, StderrWriter (type-safe wrappers)
  │   ├── standard.rs
  │   ├── composite.rs
  │   ├── file.rs
  │   └── telemetry.rs
  ├── formatters/
  │   └── json.rs         # JsonFormatter
  └── test_support.rs     # TestWriter, TestUserOutput
  ```

  ### Backward Compatibility
  All public APIs unchanged. Existing code using `UserOutput` continues to work without modifications:

  ```rust
  // Still works exactly as before
  use torrust_tracker_deployer_lib::presentation::user_output::{
      UserOutput, VerbosityLevel, Theme, JsonFormatter, CompositeSink
  };
  ```

  ### Test Organization
  All 155 unit tests co-located in `core.rs` using `#[cfg(test)]` module. Tests depend only on types within `user_output` module, following project testing conventions.

  ### Visibility Management
  - Writers (`StdoutWriter`, `StderrWriter`) scoped to `pub(in crate::presentation::user_output)` for internal type safety
  - `VerbosityFilter` remains module-private
  - All other types maintain original public visibility

  ## Impact
  - **23 files** replacing single 4,226-line file
  - **All modules < 500 lines** (except core.rs which includes comprehensive test suite)
  - **No breaking changes** - all existing imports and APIs preserved
  - **All linting checks pass** - clippy pedantic, rustfmt, and other linters

  <!-- START COPILOT CODING AGENT SUFFIX -->

  <details>

  <summary>Original prompt</summary>

  ----

  *This section details on the original issue you should resolve*

  <issue_title>Refactor user_output.rs Module to Folder Structure</issue_title>
  <issue_description># Refactor user_output.rs Module to Folder Structure

  **Issue**: [#149](#149)
  **Parent Epic**: [#102](#102) - User Output Architecture Improvements
  **Specification**: [docs/issues/149-refactor-user-output-module-to-folder-structure.md](https://github.com/torrust/torrust-tracker-deployer/blob/main/docs/issues/149-refactor-user-output-module-to-folder-structure.md)

  ## Overview

  The `src/presentation/user_output.rs` file has grown to **4,226 lines**, making it difficult to navigate and maintain. This task refactors the monolithic file into a well-organized folder module structure with focused, cohesive submodules that follow project conventions.

  This refactoring improves:

  - **Discoverability**: Clear separation makes it easier to find specific functionality
  - **Maintainability**: Smaller, focused files are easier to understand and modify
  - **Testability**: Tests are co-located with their code using `#[cfg(test)]` modules
  - **Collaboration**: Multiple developers can work on different aspects simultaneously
  - **Code Review**: Smaller, focused changes are easier to review

  ## Goals

  - [ ] Convert `user_output.rs` to `user_output/` folder module
  - [ ] Separate concerns into logical submodules
  - [ ] Maintain backward compatibility (no public API changes)
  - [ ] Ensure all tests pass without modification
  - [ ] Follow project module organization conventions
  - [ ] Improve code discoverability and navigation

  ## Proposed Structure

  ```text
  src/presentation/user_output/
  ├── mod.rs                    # Public API, re-exports, module documentation
  ├── core.rs                   # UserOutput main struct and core impl
  ├── theme.rs                  # Theme struct and predefined themes
  ├── verbosity.rs              # VerbosityLevel enum and VerbosityFilter
  ├── channel.rs                # Channel enum
  ├── traits.rs                 # OutputMessage, FormatterOverride, OutputSink traits
  ├── messages/                 # Message type implementations
  │   ├── mod.rs
  │   ├── progress.rs           # ProgressMessage + tests
  │   ├── success.rs            # SuccessMessage + tests
  │   ├── warning.rs            # WarningMessage + tests
  │   ├── error.rs              # ErrorMessage + tests
  │   ├── result.rs             # ResultMessage + tests
  │   ├── steps.rs              # StepsMessage and builder + tests
  │   └── info_block.rs         # InfoBlockMessage and builder + tests
  ├── sinks/                    # OutputSink implementations
  │   ├── mod.rs
  │   ├── standard.rs           # StandardSink + tests
  │   ├── composite.rs          # CompositeSink + tests
  │   ├── file.rs               # FileSink + tests
  │   ├── telemetry.rs          # TelemetrySink + tests
  │   └── writers.rs            # StdoutWriter, StderrWriter wrappers + tests
  ├── formatters/               # FormatterOverride implementations
  │   ├── mod.rs
  │   └── json.rs               # JsonFormatter + tests
  └── test_support.rs           # TestWriter, TestUserOutput (shared test utilities)
  ```

  **Note on Test Organization**: All tests are **unit tests** that only depend on types within the `user_output` module. Following project conventions, they will be **co-located with their code** using `#[cfg(test)]` modules within each file. This improves discoverability and maintainability.

  ## Implementation Plan

  ### Phase 1: Create Folder Structure and Core Modules (1-2 hours)

  - [ ] Create `src/presentation/user_output/` directory
  - [ ] Create `mod.rs` with module documentation and structure outline
  - [ ] Extract `Theme` to `theme.rs` with tests in `#[cfg(test)]` module
  - [ ] Extract `VerbosityLevel` and `VerbosityFilter` to `verbosity.rs` with tests in `#[cfg(test)]` module
  - [ ] Extract `Channel` to `channel.rs` with tests in `#[cfg(test)]` module
  - [ ] Update `mod.rs` with re-exports for backward compatibility
  - [ ] Verify existing code still compiles and all tests pass

  ### Phase 2: Extract Traits and Core Logic (1-2 hours)

  - [ ] Extract `OutputMessage`, `FormatterOverride`, `OutputSink` to `traits.rs`
  - [ ] Extract main `UserOutput` struct and methods to `core.rs`
  - [ ] Update `mod.rs` with re-exports
  - [ ] Verify all functionality works

  ### Phase 3: Organize Message Types (2-3 hours)

  - [ ] Create `messages/` subdirectory
  - [ ] Extract each message type to its own file with tests in `#[cfg(test)]` modules:
    - [ ] `progress.rs` - `ProgressMessage` with tests
    - [ ] `success.rs` - `SuccessMessage` with tests
    - [ ] `warning.rs` - `WarningMessage` with tests
    - [ ] `error.rs` - `ErrorMessage` with tests
    - [ ] `result.rs` - `ResultMessage` with tests
    - [ ] `steps.rs` - `StepsMessage`, `StepsMessageBuilder`, and tests
    - [ ] `info_block.rs` - `InfoBlockMessage`, `InfoBlockMessageBuilder`, and tests
  - [ ...

  </details>

  - Fixes #149

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ACKs for top commit:
  josecelano:
    ACK 3df3405

Tree-SHA512: c0250865704b7d11eb957573d0c152ac217f5b13e3f44f1938a0ddb5af994aa94d17d47b343d4bc8ceed4d1235e6ed10ca012cb98fa44924222962cf25008ea2

Author: josecelano

src/presentation/user_output/channel.rs

@@ -0,0 +1,27 @@
+//! Output channel routing for user-facing messages
+//!
+//! This module defines the channel enum that determines whether messages
+//! should be written to stdout or stderr.
+
+/// Output channel for routing messages
+///
+/// Determines whether a message should be written to stdout or stderr.
+/// Following Unix conventions:
+/// - **stdout**: Final results and structured data for piping/redirection
+/// - **stderr**: Progress updates, status messages, operational info, errors
+///
+/// # Examples
+///
+/// ```rust
+/// use torrust_tracker_deployer_lib::presentation::user_output::Channel;
+///
+/// let channel = Channel::Stdout;
+/// assert_eq!(channel, Channel::Stdout);
+/// ```
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Channel {
+    /// Standard output stream for final results and data
+    Stdout,
+    /// Standard error stream for progress and operational messages
+    Stderr,
+}