[codex] Isolate sync decode test imports on Windows

[tianmind-studio]

Summary

• Add a temporary python_multipart stub while importing routers.sync in the PCM and Opus decode unit tests, so lightweight Windows backend envs can collect the tests without the full multipart package installed.
• Ensure the tests' existing import-time stubs expose the full routers.sync import surface, even when earlier-collected tests left partial modules in sys.modules.
• Keep the changes scoped to test isolation; production sync decode code is unchanged.

Root cause

These decode tests exercise pure PCM/Opus WAL conversion helpers, but importing routers.sync also builds FastAPI routes and imports optional service helpers. In this Windows lightweight backend venv, standalone collection failed on FastAPI's multipart package check. In collection-order runs after stub-heavy tests, partial opuslib, database.conversations, utils.cloud_tasks, and utils.other.endpoints modules could block collection before the decode assertions ran.

Before this change:

• python -m pytest backend\tests\unit\test_sync_pcm_decode.py --collect-only -q --tb=short -> RuntimeError: Form data requires "python-multipart" to be installed.
• python -m pytest backend\tests\unit\test_sync_opus_decode.py --collect-only -q --tb=short -> RuntimeError: Form data requires "python-multipart" to be installed.
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_sync_pcm_decode.py backend\tests\unit\test_sync_opus_decode.py --collect-only -q --tb=short --continue-on-collection-errors -> import failures from partial sync dependencies.

Validation

• python -m pytest backend\tests\unit\test_sync_pcm_decode.py --collect-only -q --tb=short -> 23 tests collected
• python -m pytest backend\tests\unit\test_sync_opus_decode.py --collect-only -q --tb=short -> 21 tests collected
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_sync_pcm_decode.py backend\tests\unit\test_sync_opus_decode.py --collect-only -q --tb=short --continue-on-collection-errors -> 70 tests collected
• python -m pytest backend\tests\unit\test_sync_pcm_decode.py backend\tests\unit\test_sync_opus_decode.py -q --tb=short -> 44 passed
• python -m pytest backend\tests\unit\test_sync_opus_decode.py backend\tests\unit\test_sync_pcm_decode.py -q --tb=short -> 44 passed
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_sync_pcm_decode.py backend\tests\unit\test_sync_opus_decode.py -q --tb=short -> 70 passed
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_sync_opus_decode.py backend\tests\unit\test_sync_pcm_decode.py -q --tb=short -> 70 passed
• python -m pytest backend\tests\unit --collect-only -q --tb=short --continue-on-collection-errors -> still exits 1 from existing unrelated collection errors, but both sync decode files collect and no longer appear in the error summary
• python -m black --line-length 120 --skip-string-normalization backend\tests\unit\test_sync_pcm_decode.py backend\tests\unit\test_sync_opus_decode.py
• python -m py_compile backend\tests\unit\test_sync_pcm_decode.py backend\tests\unit\test_sync_opus_decode.py
• git diff --check
• scripts\pre-commit