fix(tasks): preserve terminal states against late watcher callbacks

[Jackkp0t]

Late background watchers can overwrite a cancelled task as completed or failed, and duplicate callbacks can replace a finished task's result and enqueue another notification. Allow complete() and fail() to transition only running tasks, preserving the first terminal state and its timestamp/result/error.

Adds regression coverage for both watcher callbacks after cancellation and all completed/failed callback combinations. Existing normal completion/failure paths remain covered.

Validation (Python 3.12.13, Windows, upstream a56afcc):

• Before the fix, the new regression cases fail on terminal state/result changes (6 failures including subtests).
• python -m pytest tests/utils/test_task_manager_smoke.py -k TestTaskManager -q: 12 passed, 4 subtests passed.
• Expanded TaskManager, local shell, and Windows executor suites: 34 passed, 4 subtests passed, 2 failed. Both failures also occur on the untouched upstream files: TestAgentToolTranscript.test_save_transcript_writes_message_objects_and_dicts and test_plugin_tools_are_available_without_an_explicit_shell_env.
• pytest tests -q --maxfail=1 cannot collect on Windows: tests/release/test_business_distribution.py imports Unix-only fcntl.
• Changed production file passes flake8, isort and YAPF checks; git diff --check passes.

This preserves task state only. It does not change process termination or process-group handling (the latter is covered separately in #962).

AI assistance: Codex implemented the change, ran the offline regressions and reviewed the diff. No live model calls or credentials were used in the tests.