fix(tests): resolve duplicate_service vectorized test suite mismatch and test pollution

[harshitanagpal05]

Summary

We have successfully resolved the test suite collection and runtime failures in the duplicate checking service tests.

Proposed Changes

• Unloaded conftest.py Stub Module: Dynamically removes the backend.services.duplicate_service stub injected by conftest.py from sys.modules at test startup.
• Implemented Global and Local PyTorch Mocking: Installed a global mock for the torch library to allow tests to call torch.stack() and torch.max() without failing on dummy MagicMock embeddings.
• Prevented Test Pollution: Added a teardown_method inside TestCheckDuplicate to automatically reset the shared _mock_st.util.cos_sim mock side-effects and return values between test executions.
• Corrected Multiple-Ticket Mocks to Vectorized Format: Refactored test_picks_best_match_among_multiple_tickets to model a single vectorized comparison call instead of looping individual comparisons.
  closes bug(tests): duplicate_service unit tests fail with TypeErrors & IndexErrors when running against actual vectorized service #790

Verification Results

All 24 unit tests now compile and pass cleanly in 14.48s:

backend\tests\test_duplicate_service.py ........................         [100%]
============================= 24 passed in 14.48s =============================

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[ritesh-1918]

Superb implementation, @harshitanagpal05! I've successfully resolved all conflicts in your PR and queued it for merging into gssoc.

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