Migrate fast_plan_tasks to AgentField native .harness() (#28)

[Parth-Vasave]

Changes

• What changed: Migrated the fast_plan_tasks agent in swe_af/fast/planner.py from the custom AgentAI abstraction to the native fast_router.harness() method. Updated the test suite (tests/fast/test_planner.py, tests/test_malformed_responses.py) to mock the new implementation and added fixtures in tests/conftest.py and tests/fast/conftest.py to ensure routers are correctly attached during tests.
• Why it changed: To leverage AgentField's first-class support for .harness() and remove reliance on custom execution abstractions, as part of the broader migration effort (Migrate fast/ module (fast_plan_tasks) to AgentField native .harness() #28).

Validation

• make check (778 passed, 1 skipped, 1 warning)
• Unit tests for fast_plan_tasks success and fallback paths verified.

Behavior Impact

• Backward compatible behavior change (Internal refactoring of agent execution while preserving schemas and fallback logic).

Notes

• Added a defensive try-except around fast_router attribute access in planner.py to prevent telemetry overhead from blocking execution if a router is unattached.
• The attach_fast_router fixture was added to both root and nested conftest.py to resolve RuntimeError: Router not attached which occurs when unittest.mock.patch inspects an unattached router.

[Parth-Vasave]

I have read the CLA Document and I hereby sign the CLA

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Note to Maintainers: The CLA Assistant workflow is currently failing with a configuration error:
Error: Please add a personal access token as an environment variable for writing signatures in a remote repository...

This appears to be an infrastructure issue where the workflow lacks the necessary PAT to write to the centralized Agent-Field/.github repository. All technical tests (make check) are passing locally.

[santoshkumarradha]

Hey @Parth-Vasave 👋

We just migrated our CLA setup — the old check you're seeing is from a workflow we've since removed, so you can ignore it.

Could you quickly sign the new CLA here? 👉 https://cla-assistant.io/Agent-Field/SWE-AF

Once signed, the new license/cla check should pick up automatically. Thanks for the contribution!

[Parth-Vasave]

@santoshkumarradha signed the new CLA, please reveiw when you can! :)

[santoshkumarradha]

CC @AbirAbbas

[AbirAbbas]

sorry for the delay, taking a look

[AbirAbbas]

Reviewed and tested locally. Fast planner migration to harness() works correctly — fallback path exercised end-to-end via live docker test (12/12 steps, 0 failures). LGTM.