[P1] Add real pytest suite for code2test itself

[pro-utkarshM]

Problem

tests/ contains only manual_*.py scripts — no automated suite. A test-generation tool with no tests is a credibility + regression risk.

Tasks

• Convert manual scripts into real pytest tests.
• Unit-test: intent extraction, confidence scoring, verifier output parsing, diagnosis classification, adapters (template render).
• Mock LLM calls (no live API in CI).
• Add coverage reporting; set a baseline gate.

Acceptance

pytest runs green locally and in CI with meaningful coverage of core/ and agents/.