test: add Sensors API behavior coverage

[bhudevbhanpuriya]

Summary

This PR improves API test coverage and removes a brittle source-code assertion.

Changes

• Adds behavior tests for Sensors endpoints:

  • GET /Sensors streaming response
  • GET /Sensors empty result handling
  • POST /Sensors JSON content-type validation
  • POST /Sensors created response location header
  • PATCH /Sensors(id) missing sensor response
  • DELETE /Sensors(id) missing sensor response

• Rewrites test_update_user_error_message.py to call update_user() directly instead of reading source code as text.

• Updates pytest.ini so default pytest collection includes api/tests.

Why

The previous test checked implementation text instead of runtime behavior, which could fail after harmless refactors or pass despite broken behavior. Also, Sensors endpoints had no direct test coverage.

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Before v After

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Testing

Ran syntax validation:

.venv/bin/python -m py_compile api/tests/test_sensor_endpoints.py api/tests/test_update_user_error_message.py

Fixes #170

[bhudevbhanpuriya]

Testing

I set up a clean local test environment and ran the focused tests for the changes in this PR.

Focused endpoint tests passed locally:

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These tests cover the new Sensors endpoint behavior tests and the rewritten update_user regression test.