Add ILanguageGeneratorResolver to look up a generator by id or file extension [minor]

[matt-edmondson]

Fixes #184

What was missing

Generators are registered through DI and each one has a LanguageId and a FileExtension, but nothing turns an id or an extension into a generator. Each consumer wrote its own FirstOrDefault, so they could disagree on case and on the leading dot. The CLI work in #116 has nothing to map --language py or an output file's extension onto.

Change

• New ILanguageGeneratorResolver and LanguageGeneratorResolver in Coder/Languages:
  • Generators lists the generators in registration order.
  • TryGetByLanguageId(id, out generator) ignores case.
  • TryGetByFileExtension(extensionOrPath, out generator) ignores case. It accepts py, .py, or a path such as out/model.py. A path with no extension, and an extension no generator writes, return false.
  • When two generators claim the same id or extension, the first one registered wins. This keeps the result deterministic even if duplicate registration (Calling AddCoder/AddLanguageGenerators twice registers all seven generators twice, so .Single(g => g.LanguageId == …) throws and the editor shows each language twice #150) ever comes back.
• AddLanguageGenerators, and therefore AddCoder, registers the resolver with TryAddSingleton.
• CoderEditorApp builds its preview lookup and the language list on the resolver instead of its own FirstOrDefault. Its constructor is unchanged, so Program, the UI gallery and the tests don't change.

Tests (LanguageGeneratorResolverTests)

• Every built-in id resolves to its generator, including mixed and upper case.
• Every built-in extension resolves to its generator, bare, dotted, upper-case, and as a path with / and \ separators.
• Unknown ids and extensions return false and a null generator. Cases covered: empty, ., an extensionless path, a dot in a directory name only, and model.py.bak.
• Generators lists all seven.
• With a stub pair, a duplicate id or extension resolves to the first generator. A generator that declares its extension with a leading dot still resolves.

With the registration in ServiceCollectionExtensions reverted, 33 of the 35 cases fail. The other 2 construct the resolver directly. Full Coder.Test suite: 1225 passed, 0 failed. Coder.sln builds clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_012KjGQ13RexM4WYjuEaBtE6

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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