docs(adk): Document endpoint agent resolver

[Atwolf]

Objective

Document the ADK endpoint agent resolver introduced in #1846 so the service boundary, helper convention, and routed endpoint behavior are captured in the package docs without expanding the original feature PR.

Context: @contextablemark requested this follow-up for my previous contribution #1846 Original issue: #1723.

Execution Steps

• Add a 0.7.0 changelog entry for the endpoint-level resolver hook, derived capabilities endpoint routing, /agents/state routing, and resolve_agent_from_message_history() helper convention.
• Update ARCHITECTURE.md with the FastAPI endpoint-layer boundary, resolver dispatch flow, synthetic-input behavior for non-run surfaces, continuity requirements, and tool-result affinity ownership.
• Add a USAGE.md example for one FastAPI endpoint routing across multiple ADKAgent wrappers using extractor-fed request state plus resolve_agent_from_message_history().
• Extend CONFIGURATION.md with resolver timing, default fallback semantics, trusted routing guidance, legacy extract_headers caveats, and the helper convention for tool-result resumption.

Verification and Testing

• Ran targeted ADK middleware tests: uv run pytest tests/test_endpoint_agent_resolver.py tests/test_message_history.py tests/test_utils_converters.py (150 passed).
• Review pass for documentation conventions, implementation parity, and reader semantics.

@contextablemark I didn't get a chance to thank you for your kind words on the previous PR, they are appreciated.

[Atwolf]

@contextablemark Created this PR to add the docs we mentioned - not sure why it didn't add you as a reviewer