fix: decode diagnostic file URLs before reading sources

[charan-rathore]

Description

normalizePath supports diagnostic filenames starting with file://, but currently removes the prefix without decoding the URL. A filename such as a space.ts becomes a%20space.ts. readSources cannot find that file, so its diagnostics are silently dropped. Percent signs, hashes, and non-ASCII names have the same problem.

Use Node's fileURLToPath to convert the URL to a real filesystem path, including percent decoding and platform-specific path handling. Ordinary relative and absolute paths keep their existing behavior.

The tests create real files, turn their names into file URLs, normalize them, and read their contents through readSources. All four encoded-name cases fail before this change and pass after it. A plain-path control checks that literal percent sequences in non-URL filenames are not decoded.

This completes the file-URL support discussed in #679; it does not alter the checker command or spawn handling.

Validation: all 46 unit tests, package typecheck, build, and changed-file Biome checks pass. Full repository lint has a pre-existing formatting error in untouched utils.spec.ts.

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commit: a5c4b9d

[charan-rathore]

Closing this since there has been no movement on it. Happy to reopen and pick it back up if there is interest.