Fix memory leak in CapturingToken correlation map

[Copilot]

The capturingTokenCorrelationMap was leaking entries when requests failed before reaching BYOK providers or when non-BYOK models were used.

Changes

• Added finally block in ExtensionContributedChatEndpoint.makeChatRequest2() to ensure correlation map cleanup in all code paths
• Cleanup is idempotent: if BYOK providers already retrieved the token, the finally block safely returns undefined

try {
  const response = await this.languageModel.sendRequest(...);
  // ... process response
} catch (e) {
  // ... error handling
} finally {
  // Clean up correlation map entry to prevent memory leak.
  // If the request reached a BYOK provider, they already retrieved and removed this.
  // If not (e.g., request failed early or model isn't BYOK), we need to clean it up here.
  retrieveCapturingTokenByCorrelation(ourRequestId);
}

Addresses feedback from #3474: #3474 (comment)

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.