feat(tests,config): add tests, response delay, and logging fixes

[SVilgelm]

Add comprehensive for configuration parsing and port
precedence. New main_test.go covers missing config file handling,
invalid PORT env values, using env port when no flag is provided,
flag overriding env, config+flag precedence, and decoding unknown
fields. Helpers are added to write temporary config files.

Introduce a Delay field on Response (time.Duration) and apply the
delay when writing responses. Extract JSON content type into a
constant (JsonContentType) and use it when setting the header.

Simplify responsesWriter by removing the logger parameter and use
slog.Error for error reporting when writing response bodies.

Misc: update test logger helper signature to accept testing.TB and
adjust imports to include time. These changes enable testing of
port/config behavior and add configurable response delays.

[Copilot]

Pull Request Overview

This PR refactors the main function into a testable run() function and adds comprehensive unit tests for the CLI logic. The refactoring enables testing without actually starting the HTTP server by introducing a startServer flag parameter.

• Extracts main logic into testable run() function with dependency injection for args, env, stderr, and server startup control
• Adds getEnvMap() helper to capture environment variables as a map for testing
• Introduces comprehensive unit tests covering config file handling, port precedence, and error scenarios

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
main.go	Refactors main into testable run() function with better error handling and flag parsing
main_test.go	Adds unit tests for run() function covering various CLI scenarios and error cases
integration_test.go	Adds integration test that starts real HTTP server and exercises endpoints
config.go	Introduces JsonContentType constant to reduce string literal duplication
responses_writer_test.go	Updates tests to use new JsonContentType constant instead of hardcoded strings

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[Copilot]

Pull Request Overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

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