fix: fall back to file extension for usages language with custom match patterns

[jonathannorris]

Summary

• dvc usages sets each reference's language from LANGUAGE_MAP, which only covers the built-in parser extensions. Files matched with a custom --match-pattern for any other extension (e.g. html) got language: undefined, which JSON.stringify drops, so the code usages upload fails since the API requires language.
• Fall back to the file extension when it isn't in LANGUAGE_MAP.
• Only use the extension as the language when it matches [a-z0-9]{1,16}; otherwise fall back to unknown.
• Add an HTML fixture and a test covering a custom html match pattern with --format json.

[Copilot]

🟡 Changes recommended

Prototype-inherited map entries can violate the helper’s string return contract, and the integration test uses unsafe shared state.

2 open findings

• Use isolated temporary output paths and clean them up · New
• Restrict LANGUAGE_MAP lookups to own properties · New

What changed in this PR

Adds safe language fallback handling for custom usage match patterns.

Changes:

• Maps unknown safe extensions to lowercase language names.
• Defaults unsafe extensions to unknown.
• Adds HTML fixture and unit/integration coverage.

File	Description
test-utils/​fixtures/​usages/​sample.html	Adds HTML usage fixture.
src/​utils/​parsers/​language.test.ts	Tests language mapping and fallbacks.
src/​utils/​parsers/​index.ts	Adds extension-to-language fallback.
src/​utils/​parsers/​BaseParser.ts	Applies fallback to usage references.
src/​commands/​usages/​usages.test.ts	Tests custom HTML pattern output.

🧠 Review effort: Balanced

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