feat(tests): add pytest suite for gateway, ingestion, retrieval and e…

[sabinbobu]

Add pytest suite and GitHub Actions CI

• Add tests for gateway, ingestion, retrieval and experiments
• All 11 tests passing, zero real API calls — all network mocked
• Add GitHub Actions CI workflow: ruff + mypy + pytest on every PR
• Add pytest and pytest-mock to dev dependencies
• Fix pytest Python version — now uses venv Python 3.12, not system 3.13