test: align startup removeChatRun stub

Return undefined from the early-startup test stub so it satisfies the current chat run registry removal contract during core test typechecking.

Tests:
- pnpm check:changed
- git diff --check

Co-authored-by: Codex <noreply@openai.com>

src/gateway/server-startup-early.test.ts

@@ -50,7 +50,7 @@ describe("startGatewayEarlyRuntime", () => {
       chatRunBuffers: new Map(),
       chatDeltaSentAt: new Map(),
       chatDeltaLastBroadcastLen: new Map(),
-      removeChatRun: () => {},
+      removeChatRun: () => undefined,
       agentRunSeq: new Map(),
       nodeSendToSession: () => {},
       skillsRefreshDelayMs: 30_000,