docs: fix the timeout configuration example

[Torch4084]

Summary

• fix the timeout example so the injected httpx.AsyncClient preserves FastApiMCP's in-process ASGI transport and base URL
• correct the documented default timeout from 5 seconds to 10 seconds in the FAQ
• add a configuration test that locks in the 10 second default timeout

Fixes #231

Validation

• uv run pytest --no-cov tests/test_configuration.py
• uv run ruff check examples/07_configure_http_timeout_example.py tests/test_configuration.py