fix: abort response-cancelled answer streams

[BigSimmo]

Summary

• Added cancellation hardening for /api/answer/stream so request aborts and response-body cancellation propagate through a single merged AbortSignal to both retrieval and summary generation.
• Added focused regression test in tests/private-access-routes.test.ts asserting cancellation aborts the signal passed to answerQuestionWithScope.

Verification

• Attempted focused test run: npm run test -- --run tests/private-access-routes.test.ts -t "aborts streamed answer generation when the response body is cancelled".
• Local execution is blocked by missing Vitest binary (node_modules/vitest/vitest.mjs).

Checks not run

• Full suite and provider-bound checks were not run because local dependency installation is incomplete.

Risk / follow-up

• Re-run focused and broader offline checks after installing dependencies.

[supabase]

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Review details

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between fb35cab and a647460.

📒 Files selected for processing (2)

• src/app/api/answer/stream/route.ts
• tests/private-access-routes.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/stream-cancel-hardening-retry-rebase

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