fix(a2a): honor PORT when serving locally

[notgitika]

Summary

• Resolve serve_a2a's port from the explicit argument, then PORT, then the existing 9000 default.
• Preserve explicit port= precedence for backwards compatibility.
• Document the environment fallback and cover default, environment, and explicit-port behavior.

This is stacked on #591 because the current main branch does not yet contain the A2A SDK v1 serve_a2a implementation. It enables the CLI to assign distinct ports to multiple local CodeZip A2A runtimes.

Testing

• pytest tests/bedrock_agentcore/runtime/test_a2a.py -q (28 passed)
• ruff check and ruff format --check on changed Python files
• Full suite: 2,795 passed, 10 skipped, 4 xpassed; the sole sandbox-blocked localhost integration test passed separately with socket access

[jariy17]

pretty good, just some edge cases to solve for

[jariy17]

PORT is used for uvicorn, but the generated agent card still defaults to localhost:9000. with PORT=9001, the server binds to 9001 while the card advertises 9000. can we pass the resolved port into the card URL and add a test for it?