fix(vercel): enable auto-delete session on Vercel stream release

The "delete current conversation" feature was not working on Vercel
deployment because the stream flow uses a separate lease mechanism.
The session_id created during prepare phase was not preserved for
deletion when the stream ends.

Changes:
- Add SessionID field to streamLease struct to preserve session_id
- Pass session_id to holdStreamLease during prepare
- Modify releaseStreamLease to return auth and session_id
- Call autoDeleteRemoteSession in handleVercelStreamRelease when
  releasing a lease with auto-delete mode enabled

Closes #vercel-auto-delete

internal/httpapi/openai/chat/handler.go

@@ -33,6 +33,7 @@ type Handler struct {
 
 type streamLease struct {
 	Auth      *auth.RequestAuth
+	SessionID string
 	ExpiresAt time.Time
 }