docs(a2a): document Python remote agent card interceptors

[xThreeh]

This adds a focused explanation of card_request_interceptors to the Python A2A consuming guide.

The current ADK Python implementation exposes CardRequestInterceptor and A2aCardRequestConfig for adding per-invocation headers while fetching a remote agent card over HTTP(S). The guide currently documents request interceptors for A2A messages but does not explain this separate card-fetch hook. The new section includes a small authentication-header example and clarifies that the hook is ignored for an in-memory AgentCard or a local file path.

The wording and example match the current implementation in google/adk-python, including the last-interceptor-wins behavior. The documentation build passes with mkdocs build --strict.

This covers one item from #2169 to keep the review focused. The remaining release-documentation items can be handled separately.

Fixes #2169

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