fix: resolve staticcheck nil pointer dereference warning

Use t.Fatal instead of t.Error when retrieved session is nil to prevent
subsequent nil pointer dereference on retrieved.ID.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mcp/session_test.go

@@ -85,7 +85,7 @@ func TestSessionStoreCreate(t *testing.T) {
 	// Verify we can retrieve it
 	retrieved := store.Get(session.ID)
 	if retrieved == nil {
-		t.Error("Should be able to retrieve created session")
+		t.Fatal("Should be able to retrieve created session")
 	}
 	if retrieved.ID != session.ID {
 		t.Error("Retrieved session ID should match")