TEST: Harden Copilot allocation cancellation observation - #3142

Description

Fix an unrelated test timeout that blocked #3124. This branch starts from current main, which already includes #3122; that PR hardened a different lifecycle test in this file.

The full macOS Python 3.11 dev_all job log fails at test_github_copilot_target.py:2040: asyncio.wait_for(allocated.wait(), timeout=2.0). The test had not reached request_task.cancel(...), so this failure is an allocation-readiness timeout, not evidence of failed cancellation cleanup. The log does not establish why allocation missed the two-second bound. A controlled three-second delay before allocation reproduces the same failure with working cleanup.

Use the existing wait_for_completion_async helper, with its bounded 30-second watchdog, for allocation readiness, deletion readiness, and request completion. Unlike asyncio.wait_for, an expired observer does not cancel the operation under test. Gate allocation and deletion with events, and add controlled zero-timeout observer cases. These check that allocation observation adds no caller cancellation, cleanup observation adds no second cancellation, and the original cancellation object, message, cause, retry behavior, and unrelated session remain intact. Teardown releases the gates and drains the request and observer tasks.

Only this test changes. Production behavior and component responsibilities do not change. #3124, #3138, #3139, and the Crescendo follow-up #3140 are outside this branch.

Tests and Documentation

Local validation used Windows and Python 3.12.12:

• uv sync --frozen --extra all — passed; lockfile unchanged.
• uv run --no-sync pytest tests\unit\prompt_target\target\test_github_copilot_target.py -k creation_is_cancelled_after_allocation -q — baseline passed, 3 tests.
• uv run --no-sync pytest tests\unit\prompt_target\target\test_github_copilot_target.py -q — passed, 94 tests, including all 6 updated cancellation cases.
• uv run --no-sync pre-commit run --all-files with UV_NO_SYNC=1 — all hooks passed.
• git diff --check — passed.

Controlled checks used a session-local pytest harness, not committed test tooling:

• Add await asyncio.sleep(3) before the real PromptNormalizer._calc_hash_async. Execute the original test from main: all 3 cases fail at the original allocation-readiness wait, as expected. Execute the updated test with the same delay: all 6 cases pass.
• Replace GitHubCopilotTarget._cleanup_allocated_session_async with a no-op AsyncMock: all 6 updated cases fail because deletion never starts, as expected. The watchdog change does not hide lost cleanup.

The new committed expired-watchdog cases provide deterministic checks without wall-clock sleeps. macOS/Python 3.11 validation is left to CI. Documentation and notebook execution are not applicable to this test-only change.