fix(a2a): advertise resolved port on explicit cards

[notgitika]

Summary

• update caller-supplied A2A cards with the URL resolved by serve_a2a
• preserve AGENTCORE_RUNTIME_URL precedence and direct build_a2a_app behavior when no override is supplied
• add regression coverage for PORT, explicit port=, and deployed runtime URL precedence

Reproduction

Generated ADK and LangGraph templates provide cards that advertise http://localhost:9000/. Outside agentcore dev, running:

PORT=9002 uv run python main.py

binds the server to 9002 but previously left the explicit card URL at 9000. serve_a2a already passes its resolved local URL into build_a2a_app; this change applies that URL to explicit cards as well as generated cards.

Testing

• uv run pytest tests/bedrock_agentcore/runtime/test_a2a.py -q (31 passed, A2A v1)
• A2A SDK 0.3 focused suite (30 passed, 1 skipped)
• uv run pytest tests/bedrock_agentcore/runtime -q (332 passed)
• uv run pytest tests/integration/runtime/test_a2a_integration.py -q (10 passed)
• Ruff lint and format checks
• standalone generated ADK at 9002 advertises http://localhost:9002/
• standalone generated LangGraph at 9003 advertises http://localhost:9003/

[github-actions]

✅ No Breaking Changes Detected

No public API breaking changes found in this PR.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)