Cache Faraday connection and auth tokens for improved performance

[stevenou]

Summary

• Memoize Faraday connection to reuse HTTP connections across requests (eliminates TLS handshake overhead per request)
• Use apply! instead of fetch_access_token! to leverage googleauth's built-in token caching (only refreshes when token is missing or expires within 60 seconds)

Problem

The current implementation has two performance issues:

1. Creates a new Faraday connection on every request (TLS handshake overhead)
2. Calls fetch_access_token! on every request (unnecessary network call to Google's auth service)

Solution

1. Connection memoization: The Faraday connection is now cached in @connection and reused across all requests from a single client instance
2. Token caching: Using @authorizer.apply!(request.headers) instead of manually setting the Authorization header with fetch_access_token!. The googleauth gem's apply! method has built-in token caching.

Test plan

• Specs pass
• Test with service account credentials to verify token caching works
• Test with default credentials (ADC)
• Test with API key (no auth changes needed)
• Verify SSE streaming still works correctly

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