feat(to_a2a): add service_url support to to_a2a

[kaligautier]

Summary

• add an optional service_url parameter to to_a2a() and use it for the published agent card URL
• preserve the existing host/port/protocol path as the fallback behavior and document precedence
• cover the new API with unit tests and update the A2A sample documentation

Root cause

to_a2a() only accepted split host/port/protocol inputs even though the generated agent card ultimately needed a single service URL string, forcing callers to parse environment-provided URLs themselves.

Testing

• .venv/bin/python -m pytest tests/unittests/a2a/utils/test_agent_to_a2a.py

Fixes #5060

[kaligautier]

@rohityan @xuanyang15