refactor: migrate tests to use edx-platform

[igobranco]

This commit refactors the testing infrastructure to leverage edx-platform's test configuration instead of custom mock backends, improving test reliability and reducing maintenance overhead.

Key changes:

Test Infrastructure:

• Configure pytest with 60-second timeout protection to prevent hanging tests
• Add pytest-timeout plugin to requirements/test.in
• Remove TOX dependency in favor of direct virtual environment usage

Makefile Enhancements:

• Add automatic MongoDB Docker container management (mongo:7)
• Implement virtual environment auto-creation and validation
• Add Python 3.11 version checking with clear error messages
• Create new test targets: test (auto-managed MongoDB) and test-keep-mongo
• Add MongoDB lifecycle targets: mongo-start, mongo-stop, mongo-status, mongo-ping
• Update test execution to run from edx-platform context with proper PYTHONPATH
• Add coverage reporting with combine and include filters

Settings Refactoring:

• Migrate from custom mock backends to edx-platform test settings
• Remove deprecated NAU_*_MODULE override settings from nau_openedx_extensions/settings/test.py
• Add MongoDB connection configuration for modulestore tests
• Configure CONTENTSTORE and update_module_store_settings for Mongo integration
• Remove test-specific settings from app configs (apps.py files)
• Create separate test_settings.py for coursecertificate app if needed

GitHub Actions CI:

• Add MongoDB version to test matrix for documentation
• Add Docker verification step to ensure container support
• Add 60-minute timeout to test execution
• Update checkout and requirements steps to use proper working directories
• Configure cache paths for both edx-platform and nau-openedx-extensions

Additional Improvements:

• Add detailed logging to exception handlers in partner_integration views
• Clean up code formatting and improve error messages
• Add comprehensive Makefile help documentation

This migration enables:

• Direct use of edx-platform factories and models in tests
• Real MongoDB testing via Docker containers
• Consistent test environment between local and CI
• Reduced test maintenance by depending on edx-platform test infrastructure
• Automatic timeout protection preventing indefinite test hangs

related to: fccn/nau-technical#674

[igobranco]

@Tiago-Salles I already have squashed my last commits.

[Tiago-Salles]

LGTM!