fix: delegate CloseIdleConnections through userAgentRoundTripper

[infiniteregrets]

closes #248

Summary

• userAgentRoundTripper wraps the user-provided http.Client.Transport to inject User-Agent headers, but did not implement CloseIdleConnections().
• This caused http.Client.CloseIdleConnections() to silently become a no-op after s2.New() wraps the transport, since the Go standard library only calls CloseIdleConnections() if the transport implements it.
• Added CloseIdleConnections() to userAgentRoundTripper that delegates to the underlying transport when supported.

Test plan

• Verify go build ./... passes
• Confirm CloseIdleConnections() propagates through to the underlying transport after wrapping

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR fixes a subtle but real bug where http.Client.CloseIdleConnections() silently became a no-op after s2.New() replaced the HTTP transport with a userAgentRoundTripper wrapper. The fix adds CloseIdleConnections() to userAgentRoundTripper using an idiomatic Go interface-check delegation pattern — the same approach the standard library itself uses internally.

Confidence Score: 5/5

Safe to merge — the change is minimal, correct, and follows idiomatic Go patterns for delegating optional transport methods through wrappers.

Single method addition with no logic changes, no new state, and no risk of breakage. Remaining observations are P2 and do not affect correctness.

No files require special attention.

Important Files Changed

Filename	Overview
s2/client.go	Adds CloseIdleConnections() to userAgentRoundTripper to delegate to the underlying transport, fixing a silent no-op when callers invoke http.Client.CloseIdleConnections() after the SDK wraps the transport.

_{Reviews (1): Last reviewed commit: "fix: delegate CloseIdleConnections throu..." | Re-trigger Greptile}