Fix race condition in stdio.SendRequest with canceled context (#503)

Add early context cancellation check to prevent race condition where
SendRequest could return nil instead of context.Canceled when called
with an already-canceled context. This issue only manifested in certain
environments like GitHub Actions due to timing differences.

The fix ensures consistent behavior by checking context cancellation
before any I/O operations.

🤖 Generated with [opencode](https://opencode.ai)

Co-authored-by: opencode <[email protected]>

Author: ezynda3

client/transport/stdio.go

@@ -307,6 +307,13 @@ func (c *Stdio) SendRequest(
 	ctx context.Context,
 	request JSONRPCRequest,
 ) (*JSONRPCResponse, error) {
+	// Check if context is already canceled before doing any work
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	default:
+	}
+
 	if c.stdin == nil {
 		return nil, fmt.Errorf("stdio client not started")
 	}