Harden A2A eval bug paths

[pmbstyle]

Summary

• isolate inbound A2A turns by context and mark empty A2A replies as failed
• retry provider tool calls with tool_choice=auto when thinking-mode routes reject required/object tool choice
• expose worker launcher Docker readiness through gateway_status
• avoid rewriting normal short final replies like noted/done as textual tool invocations

Tests

• uv run pytest tests\test_a2a_gateway.py tests\test_litellm_provider_payload_hardening.py tests\test_gateway_status_tool.py
• uv run pytest tests\test_heartbeat.py::test_detect_textual_tool_invocation tests\test_a2a_gateway.py tests\test_litellm_provider_payload_hardening.py tests\test_gateway_status_tool.py
• Test instance smoke: A2A direct worker summary completed, ops status reported docker/image ready, isolated context did not leak marker, short noted. reply preserved