Fix FlatConfigLoader cache behavior when configOverride is supplied

Summary

FlatConfigLoader.getConfigFor() caches a ResolvedConfig by filename before it processes configOverride.

A later call for the same filename with a different configOverride returns the cached result. This ignores the later override. The problem applies to every ConfigData field, including aria, not only aria.

Affected area

  • src/flat-config/flat-config-loader.nodejs.ts
  • FlatConfigLoader.getConfigFor()

Reproduction

const loader = new FlatConfigLoader(configFilePath);
const filename = "test.html";

const first = await loader.getConfigFor(filename, { aria: "1.2" });
const second = await loader.getConfigFor(filename, { aria: "1.3" });

expect(first.getAriaVersion()).toBe("1.2");
expect(second.getAriaVersion()).toBe("1.3"); // Currently returns "1.2" from the filename cache.

The same behavior occurs with other override properties. For example, a later configOverride.rules value can be ignored after an earlier call has cached the filename.

Required change

Do not use the filename-only cache when configOverride is supplied. Preserve the existing cache behavior for calls without configOverride. Alternatively, include a complete and stable representation of configOverride in the cache key.

Acceptance criteria

  • Two calls for the same filename with different override values return configurations that reflect their respective overrides.
  • A call with configOverride does not pollute the cache used by calls without configOverride.
  • Calls without configOverride retain filename-based caching.
  • Tests cover at least two calls for one filename with different override values.

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