test(dev-mock): raise the dev:mock provider context window to the 64K floor

Same site class as the e2e fixtures: tests-js/scripts/mock-server.ts writes
the config for `npm run dev:mock`. Entry-level providers.<name>.context_length
is honoured since #98387, so a 4096 window now fails agent init below
MINIMUM_CONTEXT_LENGTH exactly as it did for the Playwright fixtures.

tests-js/scripts/mock-server.ts

@@ -1087,7 +1087,7 @@ providers:
     key_env: MOCK_API_KEY
     models:
       mock-model: {}
-    context_length: 4096
+    context_length: 64000
 `,
     'utf8',
   )