Refactor Teradata provider tests to remove legacy unittest imports

[RyukR1]

This PR refactors the test files for the Teradata provider to fully align with Airflow's pytest migration guidelines.

Specifically, it removes:

• Unused import unittest statements
• The legacy if __name__ == "__main__": unittest.main() execution blocks from the bottom of the files

These files already utilize standard pytest decorators, asserts, and mocks, making the legacy unittest remnants obsolete.

Modified Files:

• providers/teradata/tests/unit/teradata/operators/test_bteq.py
• providers/teradata/tests/unit/teradata/utils/test_bteq_util.py
• providers/teradata/tests/unit/teradata/utils/test_encryption_utils.py

Tests Run

Executed the full Teradata provider test suite inside the local Breeze environment:

breeze testing providers-tests --test-type "Providers[teradata]"

Was generative AI tooling used to co-author this PR?

• Yes — Antigravity (Gemini 3.5 Flash)
  Generated-by: Antigravity (Gemini 3.5 Flash) following the guidelines
  EOF

[boring-cyborg]

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