fix: replace dedup resolver closure chain with array to prevent hangs

[justiniggy]

Summary

• Replace fragile closure-chain pattern in RequestDeduplicator with a simple resolvers array
• Fix removeInflight() silently dropping waiters — they now receive a 503 response instead of hanging forever

Problem

1. Closure chain is fragile and hard to reason about:

The old getInflight() chained each new waiter by capturing and wrapping the previous entry.resolve via closures:

const orig = entry.resolve;
entry.resolve = (result) => {
  orig(result);    // call previous
  resolve(result); // resolve this waiter
};

This pattern is hard to verify for correctness and makes the code unnecessarily complex.

2. removeInflight() causes waiters to hang forever (real bug):

When the original request fails (client disconnect, timeout, error), removeInflight() was called which simply deleted the inflight entry:

removeInflight(key: string): void {
  this.inflight.delete(key);  // waiters' promises never resolve!
}

Any waiters (getInflight() callers) would have promises that never resolve, causing those HTTP responses to hang indefinitely until the client times out.

Fix

• InflightEntry now holds resolvers: Array<(result) => void> instead of a single resolve function + waiters array
• complete() iterates all resolvers
• removeInflight() resolves all waiters with a 503 error response so they can retry independently

Test plan

• Verify build passes: npm run build
• Send two identical requests concurrently → second should wait for first and get same response
• Kill the first request mid-flight → second should get 503 (not hang forever)
• Send request, let it complete, send same request within 30s → should get cached response

🤖 Generated with Claude Code