Fix GCP hook: preserve httpx request extensions (timeout_ms)

[yvankondjo]

Fixes #253
Root cause
GoogleCloudBeforeRequestHook.before_request rebuilds an httpx.Request without forwarding request.extensions. httpx stores per-request timeout configuration in extensions, so timeout_ms is dropped and the default timeout is used.

Change

• Preserve request.extensions when creating next_request by passing extensions=request.extensions.

Tests

• Added regression test src/mistralai/extra/tests/test_gcp_timeout_extensions.py asserting the timeout extension is preserved through the hook.