fix(hybrid): re-raise CancelledError in platform attempt walker

[njbrake]

Description

run_platform_attempts catches BaseException around each attempt so a provider client raising outside the Exception hierarchy still falls through to the next candidate. That catch also swallows asyncio.CancelledError, which a disconnected caller produces: it gets classified as an unknown upstream error, an abandoned attempt is recorded against a provider that answered fine, and it is raised as a provider-failure HTTP exception that suppresses the cancellation the server is waiting to unwind.

This adds an except asyncio.CancelledError: raise guard ahead of the broad catch, mirroring the standalone walker fix from #492 (_attempts.py). A unit test asserts the cancellation propagates and no abandoned-attempt row is recorded.

PR Type

• Bug Fix

Relevant issues

Fixes #501

Checklist

• I understand the code I am submitting.
• I have added or updated tests that cover my change (tests/unit, tests/integration).
• I ran the Definition of Done checks locally (make lint, make typecheck, make test).
• Documentation was updated where necessary.
• If the API contract changed, I regenerated the OpenAPI spec (uv run python scripts/generate_openapi.py).

AI Usage

• AI was used for drafting/refactoring.

AI Model/Tool used: DeepSeek V4 Flash

Any additional AI details you'd like to share:

The fix and regression test mirror the standalone walker change in #492. Verified the new test fails without the guard and passes with it.

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[coderabbitai]

Walkthrough

run_platform_attempts now re-raises asyncio.CancelledError before broad exception handling. A unit test confirms cancellation propagation without provider-failure handling, fallback, or abandoned-attempt metrics.

Changes

Cancellation propagation

Layer / File(s)	Summary
Cancellation guard and regression coverage src/gateway/api/routes/_platform.py, tests/unit/test_run_platform_attempts.py	The platform attempt runner propagates asyncio.CancelledError. The regression test verifies that cancellation does not trigger provider-failure handling or abandoned-attempt metrics.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• mozilla-ai/otari#326: Both changes modify run_platform_attempts and its tests, but this change addresses cancellation propagation.

Suggested reviewers: agpituk

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change re-raises asyncio.CancelledError and adds regression coverage for propagation and missing abandoned-attempt records, satisfying issue #501.
Out of Scope Changes check	✅ Passed	The code and test changes are limited to cancellation handling in the hybrid platform attempt walker and its regression coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title uses the valid Conventional Commit type fix, uses imperative mood, clearly describes the cancellation fix, and is 63 characters long.
Description check	✅ Passed	The description explains the bug and fix, identifies the issue, documents testing, and completes the relevant checklist and AI usage sections.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/hybrid-cancelled-request-not-provider-failure

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix/hybrid-cancelled-request-not-provider-failure

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