[JS] Expose client timeout option

[mickeywu]

Is your feature request related to a problem? Please describe.
For large requests that take the LLM over 5 mins, a timeout error occurs. It appears that there is a 300000 ms timeout hardcoded in the JS library.

This is a real world issue where you may have a background running job doing deep analysis on a long document and the LLM may take over 5 mins to process (especially so if using a model like Gemini 3 Pro).

Describe the solution you'd like
Expose the client timeout option as a config option.

Describe alternatives you've considered
It's possible to chunk the request into smaller ones manually, but the reality is that sometimes if you want to attach a large PDF and ask for deeper analysis, chances are it may take longer than 5 mins for the LLM to return.

Additional context
I'm using Genkit with the Google Gen AI model plugin.

Here's the example error if the request doesn't come back after 5 mins:

>    [cause]: HeadersTimeoutError: Headers Timeout Error
>        at FastTimer.onParserTimeout [as _onTimeout] (node:internal/deps/undici/undici:7041:32)
>        at Timeout.onTick [as _onTimeout] (node:internal/deps/undici/undici:686:17)
>        at listOnTimeout (node:internal/timers:605:17)
>        at process.processTimers (node:internal/timers:541:7) {
>      code: 'UND_ERR_HEADERS_TIMEOUT'

Additional error:

Failed to fetch from https://generativelanguage.googleapis.com/v1beta/models/gemini-3-flash-preview:generateContent: fetch failed\n    at makeRequest (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/google-genai/lib/googleai/client.js:242:11)\n    at async generateContent (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/google-genai/lib/googleai/client.js:62:20)\n    at async callGemini (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/google-genai/lib/googleai/gemini.js:518:22)\n    at async /home/abc/Projects/xyz-app/node_modules/@genkit-ai/google-genai/lib/googleai/gemini.js:571:11\n    at async /home/abc/Projects/xyz-app/node_modules/@genkit-ai/core/lib/action.js:148:27\n    at async /home/abc/Projects/xyz-app/node_modules/@genkit-ai/core/lib/tracing/instrumentation.js:91:24\n    at async runInNewSpan (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/core/lib/tracing/instrumentation.js:63:10)\n    at async actionFn.run (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/core/lib/action.js:104:18)\n    at async dispatch (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/core/lib/action.js:55:24)\n    at async wrapped.run (/home/abc/Projects/xyz-app/node_modules/@genkit-ai/core/lib/action.js:76:22) {\n  traceId: 'b8c4ecd0610eaef2a9fca6e09f47f2af',\n  ignoreFailedSpan: true\n}"}