feat(python-adk): honor isolateSessions for remote agent tools

[Whxuan0701]

Summary

• honor the existing isolate_sessions configuration in the Python remote A2A tool runtime
• mint a fresh sub-agent context for each isolated invocation while preserving shared-session defaults
• return the actual invocation context in completed and HITL-pending responses

This is a Python runtime follow-up to the accepted Go implementation in #2153. It does not change the CRD, UI, lineage headers, concurrency limits, or HITL resume routing.

Testing

• cd python && uv run pytest -q packages/kagent-adk/tests/unittests/test_remote_a2a_tool.py packages/kagent-adk/tests/unittests/test_proxy_integration.py (40 passed)
• cd python && uv run ruff check packages/kagent-adk/src/kagent/adk/_remote_a2a_tool.py packages/kagent-adk/src/kagent/adk/types.py packages/kagent-adk/tests/unittests/test_remote_a2a_tool.py packages/kagent-adk/tests/unittests/test_proxy_integration.py
• git diff --check

Risk / Notes

• isolate_sessions=false remains the default and reuses one context ID for stateful sub-agents.
• Isolated calls use a local per-call ID, so concurrent fan-out does not share mutable session state.
• Existing upstream/dependency warnings remain in the test output; no new failures were introduced.

Follow-up to #2153.

[supreme-gg-gg]

lgtm, thanks! Can you also backport this to release/0.10.x branch