test: add Python unit tests and CI workflow

[dolepee]

Summary

This PR adds 30 unit tests for the Python codebase and a GitHub Actions CI workflow to run them automatically.

Problem

The repository has zero Python test coverage. The existing CI workflow only runs Foundry/Forge tests for the Solidity contracts, leaving the Python code (chainmail.py, chainlink.py) entirely untested.

Changes

Test files:

• tests/test_chainmail.py — 15 tests covering:

  • get_content_string(): valid roundtrip, unicode, empty input, invalid base64/zlib error handling
  • get_verification_url(): URL format, parameter encoding, roundtrip with get_content_string()
  • modify_signature_header(): replacement, no-match passthrough
  • PGP_sign_message() and verify_signature(): GPG calls (mocked)
  • send_email(): end-to-end integration with mocked GPG
  • email_send_message(): stub behavior

• tests/test_chainlink.py — 15 tests covering:

  • hash(), hash_email_address(), hash_message(): determinism, case-insensitivity, whitespace normalization
  • parse_cast_send_output(): success/failure parsing of cast send output
  • is_cast_and_send_succeed(): boolean result verification
  • get_chainmail_address(): missing file and existing file scenarios

CI workflow:

• .github/workflows/python-tests.yml — runs pytest tests/ -v on push/PR to master across Python 3.10, 3.11, and 3.12

Dependencies:

• Added pytest and pytest-mock to requirements.txt

Design decisions

• External dependencies (GPG, blockchain/cast, filesystem) are mocked to keep tests fast and isolated
• Tests use pytest class-based grouping for readability
• The CI matrix covers three Python versions for broad compatibility