Add set_version encoding tests and fix missing encoding in provider writes

[Copilot]

Description

Adds tests for set_version() methods to verify encoding behavior during version updates (the cz bump path). While adding tests, discovered multiple providers weren't using configured encoding when writing files.

Tests Added

• test_json_provider_set_version_uses_encoding_with_encoding_fixture - Verifies JsonProvider writes non-ASCII content correctly
• test_toml_provider_set_version_uses_encoding_with_encoding_fixture - Verifies TomlProvider writes non-ASCII content correctly

Both tests use existing fixtures with Cyrillic characters and verify version updates preserve non-ASCII content.

Encoding Bugs Fixed

• JsonProvider.set_version() - Now uses encoding parameter and ensure_ascii=False
• TomlProvider.set_version() - Now uses encoding parameter
• UvProvider.set_lock_version() - Added encoding for pyproject.toml and uv.lock I/O
• CargoProvider.set_lock_version() - Added encoding for Cargo.toml and Cargo.lock I/O
• NpmProvider - Added _get_encoding() method; applied to all file operations (package.json, package-lock.json, npm-shrinkwrap.json)

Checklist

• I have read the contributing guidelines

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Generated-by: GitHub Copilot following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken internal documentation links (internal or external)

Expected Behavior

Version files containing non-ASCII characters (e.g., UTF-8 with Cyrillic, emoji, CJK) should maintain encoding integrity through get_version() and set_version() operations when encoding is configured.

Steps to Test This Pull Request

1. Run python -m pytest tests/providers/test_base_provider.py -v
2. Verify all 10 tests pass, including the two new set_version tests
3. Run python -m pytest tests/providers/test_npm_provider.py tests/providers/test_cargo_provider.py -v
4. Verify all existing provider tests still pass

Additional Context

Addresses feedback from #1857 review comment requesting set_version() test coverage. The encoding bugs were latent issues - existing get_version() tests only caught half the problem.

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