Expand proxy cache/import edge-case coverage in proxy.test.js

[Copilot]

This PR adds the next set of high-value tests to improve coverage around cache metadata compatibility and import argument parsing edge cases. The focus is on behaviors that were previously unverified but impact cache correctness and operator workflows.

• Cache hit: legacy metadata backfill path

  • Adds coverage for cached entries missing proxy-kutti-orig-request.
  • Verifies cacheHit serves the cached response and asynchronously backfills request metadata (including cache-date and sanitized state).

• Import CLI parsing: flag ordering and incomplete override

  • Adds coverage for --content-type appearing before positional args.
  • Adds coverage for --content-type without a value, ensuring import falls back to inferred MIME type from filename extension instead of writing an invalid override.

• Test stability improvement for async metadata writes

  • Replaces fixed-delay waiting with predicate polling to avoid timing-sensitive flakes while asserting metadata backfill.

await importIntoCache(['--content-type', 'application/custom-tgz', importUrl, srcFile]);
const { cachedFile } = computeCacheDetails('https', 'GET', importUrl);
const metaData = JSON.parse(fs.readFileSync(`${cachedFile}.meta`, 'utf8'));
assert.strictEqual(metaData.headers['content-type'], 'application/custom-tgz');