docs(sdk): document client message context

[simonrosenberg]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Document the client_context API introduced by software-agent-sdk#4110, including local and remote examples.
• Explain persistence in MessageEvent.extended_content, runtime-change reinjection, and the security boundary.
• Update SDK navigation, API/OpenAPI references, and the generated LLM context indexes.

Canvas consumer: OpenHands/agent-canvas#1782.

Validation: docs.json and openapi/agent-sdk.json both pass python3 -m json.tool; git diff --check passes.

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