Add unit test suite and GitHub Actions CI

[AliZakaee]

Add a comprehensive pytest suite (262 tests) covering pure logic, the SQLite data layer, the service layer, and the Telegram handler pipeline:

• Tooling: pyproject.toml (pytest + coverage config), requirements-dev.txt, and a root conftest.py with shared fixtures (temp DB, fake Config, clean env, duck-typed Message/CallbackQuery, stub bot, fernet key).
• Pure logic: utils (escaping, splitting, sensitive detection, masking, time), config validators, keyboards callback-data limits, embeddings (cosine/keyword/matrix), KB chunking/context.
• Data layer: settings coercion round-trips and full DAL incl. approval double-tap atomicity, FK cascade, owner-pending TTL — against a real temp DB.
• Services: OpenAI (Fernet protect/unprotect, _map_error, max_tokens retry), Codex (prompt build, flag fallback, subprocess mocked), prompt assembly, rate-limit windows, KB retrieval, memory ordering/trim, business pause expiry, file sanitization. No network or subprocess is ever touched.
• Handlers: process_business_message branch-by-branch, approval card/edit flow, telegram_api send adapter + fallback, notify_owner.

CI: .github/workflows/tests.yml runs the suite on a Python 3.10/3.11/3.12 matrix for every push and PR to main, with coverage measured and uploaded as an artifact (report-only) and an aggregate "tests" check for branch protection. README documents local runs and the merge-protection setup.