Handle GitHub reader cache read failures

[sarthakagrawal927]

Summary

• Treat GitHub-reader Cache API read failures as cache misses so a bounded, unauthenticated GitHub read can still serve the public request.
• Preserve the existing stale-copy fallback and upstream 503 behavior.
• Add regressions for cache-read recovery, stale serving during GitHub rate limiting, and retaining 503 when no usable copy exists.

Evidence

Forced caches.default.match() rejection reproduced an uncaught 500 before the fix. This confirms a cache-read error-handling defect. It does not establish that the five historical aggregate 5xx responses were caused by this defect; their exact status and cause remain unavailable from route-level rollups.

Verification

• pnpm exec vitest run test/worker.test.ts -t 'cache read failure|fresh-cache read fails|both cache reads'
• pnpm check (65 tests, format, lint, typecheck, client asset check, and Wrangler dry-run build)

References #21. No deployment performed.