Add Python test-function discovery miner (#127)

[Dimwiddle]

Description

Implement the Python discovery miner for test functions, including typed metadata extraction, framework-aware confidence, and parse-failure resilience. This wires the miner into default discovery execution and documents the behavior in the discovery feature spec.

Type of Change

• New feature (non-breaking change that adds functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have added tests that prove my feature works
• New and existing unit tests pass locally with my changes

Testing

• ruff check src/ tests/ examples/
  All checks passed!
  mypy src/
  Success: no issues found in 83 source files
  black --check src/ tests/ examples/
• pytest tests/ -v -rs

Related Issues

Closes #127