fix: share language matcher with plain diff provider

[apoorvdarshan]

Summary

• replace PlainDiffGitProvider's private extension inversion with the shared build_language_file_matcher
• preserve case-sensitive extensions, full filename rules, and multipart suffix matching in plain-diff mode
• add regression coverage for .C and Dockerfile, as requested in the issue

Why

The private implementation lowercased extensions and relied on os.path.splitext(). That classified .C as C instead of C++, dropped full filenames such as Dockerfile, and truncated multipart suffixes. Reusing the shared matcher keeps plain-diff behavior aligned with the other local providers.

Fixes #2894

Testing

• PYTHONPATH=. uv run pytest tests/unittest -q — 2,758 passed, 1 skipped, 1 xfailed
• uv run pre-commit run --files pr_agent/git_providers/plain_diff_provider.py tests/unittest/test_plain_diff_provider.py — passed
• uv run ruff check --fix pr_agent/git_providers/plain_diff_provider.py tests/unittest/test_plain_diff_provider.py — passed

[apoorvdarshan]

/agentic_review

[qodo-code-review]

Code Review by Qodo

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[IsmaelMartinez]

Approving and merging as-is.

The revert probe confirms the test earns its place: restore main's plain_diff_provider.py and it goes red, returning {'C': 100.0} instead of {'C++': 50.0, 'Dockerfile': 50.0}.

The payoff runs deeper than the token counts. On main a diff of upper.C and Dockerfile reported C at 100 percent and then sorted both files into "Other", so the language it advertised and the buckets it built disagreed. They now come from the same matcher.

Thanks for turning this round within half an hour of the issue going up, and for adding the regression I asked for.