fix(proxy): attach usage-recording listeners before streaming responses (#1222)

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Summary

Attaches the res.once('finish') and res.once('close') usage-recording listeners in src/routes/proxyRoutes.ts before streaming or sending the upstream response body so synchronous or fast-completing responses never miss the finish event and aborted streams are not billed.

Closes #1222

Key Changes

1. Listener Ordering & Stream Error Guard (src/routes/proxyRoutes.ts):
   • Registered res.once('finish', recordUsageOnce) and res.once('close', ...) immediately after setting status/headers and before starting the body stream (await pump() / res.send(text)).
   • Guarded recordUsageOnce with !res.writableEnded || streamErrored so mid-stream upstream errors or prematurely closed client connections never record billable usage.
   • Logged billing.chargeUsage failures with full reconciliation context (userId, apiId, endpointId, requestId, amountUsdc).
2. Streaming & Abort Tests (src/__tests__/usageMetering.test.ts):
   • Added tests verifying that multi-chunk streamed responses record usage exactly once and mid-stream aborted responses do not record usage.

Verification

• Verified with npx jest src/__tests__/usageMetering.test.ts (13/13 passed).

Payout Wallet (Base Mainnet USDC)

0x3806cd592a508d1992Ed007C2F53de251C2E7452