fix(companion): validate waitForIdle timeout option

docs/plans/state.json

@@ -938,6 +938,18 @@
       ],
       "test_status": "pnpm test:run src/companion/runtimeClient.test.ts src/companion/platformClients.test.ts src/companion/heartbeatLoop.test.ts src/companion/platformClients.integration.test.ts + pnpm typecheck passing"
     },
+    "companion-runtime-wait-for-idle-timeout-validation": {
+      "status": "completed",
+      "date": "2026-02-17",
+      "updated": "2026-02-17",
+      "summary": "Added `waitForIdle()` timeout validation guardrails (`timeoutMs > 0`) with regression tests for deterministic option validation behavior.",
+      "files_modified": [
+        "src/companion/runtimeClient.ts",
+        "src/companion/runtimeClient.test.ts",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/companion/runtimeClient.test.ts src/companion/platformClients.test.ts src/companion/heartbeatLoop.test.ts src/companion/platformClients.integration.test.ts + pnpm typecheck passing"
+    },
     "browser-tools-activation-clarity": {
       "status": "completed",
       "date": "2026-02-17",

src/companion/runtimeClient.test.ts

@@ -661,6 +661,16 @@ describe('CompanionRuntimeClient', () => {
     );
   });
 
+  it('waitForIdle validates timeoutMs option', () => {
+    const client = new CompanionRuntimeClient({
+      url: 'ws://127.0.0.1:1',
+    });
+
+    expect(() => client.waitForIdle({ timeoutMs: 0 })).toThrow(
+      'timeoutMs must be a positive number',
+    );
+  });
+
   it('waitForIdle resolves after pending event waiters are cleared', async () => {
     const client = new CompanionRuntimeClient({
       url: 'ws://127.0.0.1:1',

src/companion/runtimeClient.ts

@@ -614,6 +614,10 @@ export class CompanionRuntimeClient {
   }
 
   waitForIdle(options?: WaitForIdleOptions): Promise<void> {
+    const timeoutMs = options?.timeoutMs ?? this.requestTimeoutMs;
+    if (!Number.isFinite(timeoutMs) || timeoutMs <= 0) {
+      throw new Error('timeoutMs must be a positive number');
+    }
     const pollIntervalMs = options?.pollIntervalMs ?? 25;
     if (!Number.isFinite(pollIntervalMs) || pollIntervalMs <= 0) {
       throw new Error('pollIntervalMs must be a positive number');
@@ -621,7 +625,6 @@ export class CompanionRuntimeClient {
     if (!this.hasPendingWork) {
       return Promise.resolve();
     }
-    const timeoutMs = options?.timeoutMs ?? this.requestTimeoutMs;
     const signal = options?.signal;
 
     return new Promise<void>((resolve, reject) => {