docs: improve CONTRIBUTING.md to more closely match CI execution (#963)

* docs: improve CONTRIBUTING.md with stricter CI check commands

Updated the pre-submission checklist to match CI requirements more closely:
- Added --check flag to cargo fmt to verify formatting without modifying files
- Added -D warnings flag to cargo clippy to treat warnings as errors
- Added --all flag to cargo test to run tests across all workspace members
- Added lockfile check command (cargo check --locked)
- Included note about CI testing with multiple feature combinations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Remove unnecessary check after `cargo fmt` instructions

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

* docs: clarify CI checks and update formatting/lint/test commands in CONTRIBUTING.md

Rework the "To make the CI gods happy" section to mirror CI behavior:
add explanatory preamble and section headers, update the Clippy command to
use `--locked --all-targets --all-features`, switch tests to the
`cargo nextest run --all --all-features` invocation, and add notes about
the flags and nextest runner.

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

Author: 3 people

CONTRIBUTING.md

@@ -32,22 +32,39 @@ We follow the standard rust formatting style and conventions suggested by [clipp
 
 ### To make the CI gods happy
 
-Before submitting a PR make sure to run:
+> The commands below reflect what CI runs. Not all flags are always necessary for local development (e.g. feel free to omit `--all-features` or `--all-targets` during iteration), but running these exact commands before opening a PR ensures you match CI behavior.
 
-- for formatting
+Before opening a PR, run these locally:
+
+- Format:
 
   ```shell
   cargo fmt --all
   ```
 
-- the clippy lints
+  > `--all` formats all packages in the workspace.
+
+- Lint (Clippy):
 
   ```shell
-  cargo clippy
+  cargo clippy --locked --all-targets --all-features
   ```
 
-- the test suite
+  > `--all-targets` checks lib, bins, tests, examples, and benches. 
+  > 
+  > `--all-features` enables all optional features. 
+  >
+  > `--locked` ensures `Cargo.lock` is in sync with `Cargo.toml`.
+  
+  > Note: CI treats Clippy warnings as errors.
+
+
+
+- Tests:
+
+> CI uses [cargo-nextest](https://nexte.st/) for its test runner. See the [installation instructions](https://nexte.st/docs/installation/pre-built-binaries/).
 
   ```shell
-  cargo test
+  cargo nextest run --all --all-features
   ```
+