test(esc): pin async-subagent abort-controller isolation invariants

[ericleepi314]

Summary

• Critic on PR fix(esc): override cancelled tool_result with REJECT_MESSAGE in production path #150 flagged a defensive concern: when the user presses ESC and then types please resume, the engine's reset_abort_controller swaps in a fresh controller. If a subagent spawned in the previous turn held a stale (aborted) parent controller snapshot, the REJECT_MESSAGE override in _dispatch_single_tool would fire for every tool the subagent ran — spurious user-rejected results.
• Verified: no bug exists in current Python. The code at src/agent/run_agent.py:281-287 already mirrors TS typescript/src/tools/AgentTool/runAgent.ts:533-541. Async agents get a fresh, unlinked AbortController(); sync agents share params.parent_context.abort_controller (a current reference, mutated in place by engine.reset_abort_controller — no snapshot staleness possible).
• This PR is pure regression-test coverage. No production-code change. The 4 tests pin the invariants so a future refactor that switches the async path to a child-linked controller (which WOULD propagate parent abort and re-introduce the bug) is caught immediately.

Tests

1. test_async_subagent_context_has_fresh_unlinked_controller — async path: explicit override IS the fresh instance, parent abort does NOT propagate.
2. test_sync_subagent_shares_parent_controller_reference — sync path: child.abort_controller IS the parent's controller (same object).
3. test_async_subagent_unaffected_by_parent_reset — end-to-end: parent ESC + parent controller swap (simulating engine reset), async subagent controller remains untouched.
4. test_create_subagent_context_default_path_is_child_linked — default fallback: parent-abort propagates down (one-way); child-abort does NOT propagate up.

TS parity

Path	TS reference	Python mirror
Async agent	runAgent.ts:533-541 → new AbortController()	run_agent.py:281-284 → AbortController()
Sync agent	runAgent.ts:537 → toolUseContext.abortController	run_agent.py:287 → params.parent_context.abort_controller
Fallback	forkedAgent.ts:354 → createChildAbortController(parent)	subagent_context.py:86 → create_child_abort_controller(parent.abort_controller)

Test plan

• 4/4 new tests pass.
• 0 production-code changes; this is pure coverage.
• Confirmed via reading source that the tested invariants reflect actual production behavior — the tests would fail if anyone broke the parity.

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