test(a2a): Prevent peer action allow-list regressions

[ch4n3-yoon]

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:

_extract_event_actions rebuilds only escalate and skip_summarization from a remote peer's
response metadata; the other twelve EventActions fields are dropped.
test_peer_supplied_actions_cannot_mutate_caller_session asserts five of those twelve
(state_delta, artifact_delta, transfer_to_agent, agent_state,
rewind_before_invocation_id), and no test checks the allow-list contract itself. Two regressions
therefore pass today:

• adding one of the seven unasserted fields to _PEER_SETTABLE_ACTION_FIELDS — e.g. route;
• dropping one of the accepted spellings. EventActions sets populate_by_name=True, so a peer can
  send either skip_summarization or skipSummarization and both have to stay listed.

Solution:

• Assert all twelve dropped fields in the converter test, across all four converters.
• Validate the raw metadata payload before the assertions and require every denied field to come back
  non-default. Without that step a field could assert clean because schema drift made its value
  invalid, rather than because the allow-list filtered it out.
• Add test_peer_settable_action_fields_are_exactly_inert, pinning _PEER_SETTABLE_ACTION_FIELDS to
  exactly the two inert fields and their aliases.

Tests only; no behavior change.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.

pytest tests/unittests/a2a/ → 407 passed, 19 skipped (406 passed before this change).
pyink --check and isort --check-only are clean on the changed file.

Both regressions above were mutation-checked. Each mutation was applied to
_PEER_SETTABLE_ACTION_FIELDS, then the peer tests were run against main and against this branch:

mutation	on main	with this change
add "route" to the allow-list	passes	test_peer_supplied_actions_cannot_mutate_caller_session and test_peer_settable_action_fields_are_exactly_inert fail
remove "skip_summarization", keeping only the camelCase alias	passes	test_peer_settable_action_fields_are_exactly_inert fails

Manual End-to-End (E2E) Tests:

Not applicable — tests only, no runtime behavior change.