fix: disable plugins in SDK integration tests (#16036)

## Why

The TypeScript SDK tests create a fresh `CODEX_HOME` for each Jest case
and delete it during teardown. That cleanup has been flaking because the
real `codex` binary can still be doing background curated-plugin startup
sync under `.tmp/plugins-clone-*`, which races the test harness's
recursive delete and leaves `ENOTEMPTY` failures behind.

This path is unrelated to what the SDK tests are exercising, so letting
plugin startup run during these tests only adds nondeterministic
filesystem activity. This showed up recently in the `sdk` CI lane for
[#16031](https://github.com/openai/codex/pull/16031).

## What Changed

- updated `sdk/typescript/tests/testCodex.ts` to merge test config
through a single helper
- disabled `features.plugins` unconditionally for SDK integration tests
so the CLI does not start curated-plugin sync in the temporary
`CODEX_HOME`
- preserved other explicit feature overrides from individual tests while
forcing `plugins` back to `false`
- kept the existing mock-provider override behavior intact for
SSE-backed tests

## Verification

- `pnpm test --runInBand`
- `pnpm lint`

sdk/typescript/tests/testCodex.ts

@@ -44,33 +44,43 @@ export function createTestClient(options: CreateTestClientOptions = {}): TestCli
       codexPathOverride: codexExecPath,
       baseUrl: options.baseUrl,
       apiKey: options.apiKey,
-      config: mergeTestProviderConfig(options.baseUrl, options.config),
+      config: mergeTestConfig(options.baseUrl, options.config),
       env,
     }),
   };
 }
 
-function mergeTestProviderConfig(
+function mergeTestConfig(
   baseUrl: string | undefined,
   config: CodexConfigObject | undefined,
 ): CodexConfigObject | undefined {
-  if (!baseUrl || hasExplicitProviderConfig(config)) {
-    return config;
-  }
+  const mergedConfig: CodexConfigObject | undefined =
+    !baseUrl || hasExplicitProviderConfig(config)
+      ? config
+      : {
+          ...config,
+          // Built-in providers are merged before user config, so tests need a
+          // custom provider entry to force SSE against the local mock server.
+          model_provider: "mock",
+          model_providers: {
+            mock: {
+              name: "Mock provider for test",
+              base_url: baseUrl,
+              wire_api: "responses",
+              supports_websockets: false,
+            },
+          },
+        };
+  const featureOverrides = mergedConfig?.features;
 
-  // Built-in providers are merged before user config, so tests need a custom
-  // provider entry to force SSE against the local mock server.
   return {
-    ...config,
-    model_provider: "mock",
-    model_providers: {
-      mock: {
-        name: "Mock provider for test",
-        base_url: baseUrl,
-        wire_api: "responses",
-        supports_websockets: false,
-      },
-    },
+    ...mergedConfig,
+    // Disable plugins in SDK integration tests so background curated-plugin
+    // sync does not race temp CODEX_HOME cleanup.
+    features:
+      featureOverrides && typeof featureOverrides === "object" && !Array.isArray(featureOverrides)
+        ? { ...featureOverrides, plugins: false }
+        : { plugins: false },
   };
 }