fix(agent): fix data races, add tests, and fix raw protocol detection

- Fix data race on output strings.Builder in codex backend by adding
  mutex and waiting for reader goroutine before reading final output
- Fix data race on onTurnDone by initializing it before reader starts
- Fix bug where notificationProtocol zero value "" never matched
  "unknown", silently dropping all raw v2 notifications from codex
- Add round-robin polling to prevent runtime starvation in poll loop
- Log errors in claude handleControlRequest instead of silently dropping
- Add 35 tests for pkg/agent covering claude parsing, codex JSON-RPC,
  protocol detection, event handling, and helper functions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/agent/claude.go

@@ -244,10 +244,13 @@ func (b *claudeBackend) handleControlRequest(msg claudeSDKMessage, stdin interfa
 
 	data, err := json.Marshal(response)
 	if err != nil {
+		b.cfg.Logger.Printf("[claude] failed to marshal control response: %v", err)
 		return
 	}
 	data = append(data, '\n')
-	_, _ = stdin.Write(data)
+	if _, err := stdin.Write(data); err != nil {
+		b.cfg.Logger.Printf("[claude] failed to write control response: %v", err)
+	}
 }
 
 // ── Claude SDK JSON types ──