[Flaky Test] e2e-mock: subtasks fast-child fixture shadows parent-resume fixture

[zoomote]

Observed in CI run https://github.com/Zoo-Code-Org/Zoo-Code/actions/runs/30098606515/job/89498747168 (merge queue for PR #996, unrelated diff; re-run of the same SHA passed).

Roo Code Subtasks > child completing on its first response returns to parent failed with the parent completing as "Fast child completed" instead of "Fast parent resumed" — aimock served the fast-child fixture for the parent's resume request.

Two compounding weaknesses in apps/vscode-e2e/src/fixtures/subtasks.ts:

1. The fast-child fixture matches userMessage: /SUBTASK_CHILD_IMMEDIATE_COMPLETION/ with no parent-marker exclusion, but SUBTASK_FAST_PARENT_PROMPT embeds that child marker verbatim — the same collision class fixed for the cross-profile fixtures in [Flaky Test] e2e-mock: subtasks cross-profile fixture collision + task identity prompt timeout #561.
2. The parent-resume fixture guarded on the literal new_task tool-call id (call_subtasks_fast_parent_new_task_001), which validateAndFixToolResultIds can rewrite when the parent history is reopened (ClineProvider.reopenParentFromDelegation), causing the guard fixture to miss and the looser child fixture to win.

Same structural gap exists for the SUBTASK_CHILD bare fixture and the tool-call-id guards on the regular and interrupt parent-resume fixtures.