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/agent_test.go

@@ -0,0 +1,53 @@
+package agent
+
+import (
+	"context"
+	"testing"
+)
+
+func TestNewReturnsClaudeBackend(t *testing.T) {
+	t.Parallel()
+	b, err := New("claude", Config{ExecutablePath: "/nonexistent/claude"})
+	if err != nil {
+		t.Fatalf("New(claude) error: %v", err)
+	}
+	if _, ok := b.(*claudeBackend); !ok {
+		t.Fatalf("expected *claudeBackend, got %T", b)
+	}
+}
+
+func TestNewReturnsCodexBackend(t *testing.T) {
+	t.Parallel()
+	b, err := New("codex", Config{ExecutablePath: "/nonexistent/codex"})
+	if err != nil {
+		t.Fatalf("New(codex) error: %v", err)
+	}
+	if _, ok := b.(*codexBackend); !ok {
+		t.Fatalf("expected *codexBackend, got %T", b)
+	}
+}
+
+func TestNewRejectsUnknownType(t *testing.T) {
+	t.Parallel()
+	_, err := New("gpt", Config{})
+	if err == nil {
+		t.Fatal("expected error for unknown agent type")
+	}
+}
+
+func TestNewDefaultsLogger(t *testing.T) {
+	t.Parallel()
+	b, _ := New("claude", Config{})
+	cb := b.(*claudeBackend)
+	if cb.cfg.Logger == nil {
+		t.Fatal("expected non-nil logger")
+	}
+}
+
+func TestDetectVersionFailsForMissingBinary(t *testing.T) {
+	t.Parallel()
+	_, err := DetectVersion(context.Background(), "/nonexistent/binary")
+	if err == nil {
+		t.Fatal("expected error for missing binary")
+	}
+}