Add comprehensive Python testing infrastructure

[llbbl]

Summary

• Set up Poetry as the package manager with comprehensive dependency management
• Added complete pytest testing infrastructure with coverage reporting (80% threshold)
• Created organized test directory structure with unit and integration test separation
• Implemented comprehensive shared fixtures for NeRF-specific testing scenarios
• Added validation tests to verify the testing infrastructure functionality

Test plan

• [x] Install dependencies with Poetry (poetry install)
• [x] Run all tests (poetry run pytest)
• [x] Verify unit tests work (poetry run pytest -m unit)
• [x] Verify integration tests work (poetry run pytest -m integration)
• [x] Confirm coverage reporting generates HTML and XML outputs
• [x] Test shared fixtures for rays, points, configs, and mock models
• [x] Validate pytest markers (unit, integration, slow) function correctly

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