infra: Move unit tests to a subdirectory to allow space for integration tests

[malexw]

As part of a larger series of testing improvements, this change moves all the current python tests from backend/tests to the subdirectory backend/tests/unit. This gives us room to add a directory for integration tests with their own config.

AI Description

This PR introduces changes to the Makefile and updates several test files.

Makefile

• Adds new targets: watch, up, down, run-unit-tests, run-community-tests, run-integration-tests, attach, exec-backend, exec-db, migration, migrate, downgrade, reset-db, setup, setup-use-community, win-setup, lint, first-run, win-first-run, generate-client-web, and install-web.
• Updates the run-tests target to include run-unit-tests.
• Removes the run-tests target's dependency on build.

Test Files

• Moves several test files from src/backend/tests/ to src/backend/tests/unit/: configuration.yaml, conftest.py, test_deployments.py, test_agent.py, test_agent_tool_metadata.py, test_blacklist.py, test_citation.py, test_conversation.py, test_document.py, test_file.py, test_message.py, test_organization.py, test_snapshot.py, test_tool_call.py, and test_user.py.
• Updates import statements in the moved test files to use the new path.
• Removes the src/backend/tests/factories/ and src/backend/tests/model_deployments/mock_deployments/ directories and their contents.
• Adds a new file, src/backend/tests/unit/factories/__init__.py, containing the same contents as the removed src/backend/tests/factories/__init__.py.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.30769% with 7 lines in your changes missing coverage. Please review.

Project coverage is 77.10%. Comparing base (4edacee) to head (a1172aa).
Report is 139 commits behind head on main.

Files	Patch %	Lines
...ests/unit/services/auth/test_request_validators.py	90.69%	4 Missing ⚠️
src/backend/tests/unit/tools/test_lang_chain.py	0.00%	2 Missing ⚠️
src/backend/tests/unit/conftest.py	98.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
- Coverage   81.38%   77.10%   -4.29%     
==========================================
  Files         199      238      +39     
  Lines        8354    10455    +2101     
==========================================
+ Hits         6799     8061    +1262     
- Misses       1555     2394     +839     

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