Flaky Agent Host E2E — Copilot test: custom subagent permits requested file output in its system instructions

https://github.com/microsoft/vscode/actions/runs/34468840657/job/102848658479?pr=335447

Agent Host E2E failure details:

===== Agent Host E2E — Copilot failure =====
1 failing

1) Agent Host E2E — Copilot
custom subagent permits requested file output in its system instructions:

+ expected - actual

{
"childResponse": "CUSTOM_AGENT_CHILD_OK"
"childTurns": [
"complete"
- "complete"
]
"parentResponse": "PARENT_DONE"
}

AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
+ actual - expected

{
childResponse: 'CUSTOM_AGENT_CHILD_OK',
childTurns: [
'complete',
+ 'complete'
],
parentResponse: 'PARENT_DONE'
}

at Context. (file:///Users/runner/work/vscode/vscode/out/vs/platform/agentHost/test/node/e2e/suites/subagentSuite.js:159:14)
at process.processTicksAndRejections (node:internal/process/task_queues:104:5)

Agent Host E2E — Copilot failed with code 1
Error: Process completed with exit code 1.