PDFBOLT-918 Harden Python SDK typing and CI

[mszymanowski]

Summary

• add exported TypedDict request surfaces for direct/sync/async conversion params and options
• type public helper kwargs with Unpack[...] while preserving existing runtime request mapping
• align conversion result typings with the Node SDK: sync results are SUCCESS/isAsync=False, webhook events are SUCCESS | FAILURE/isAsync=True, and exported error-code literals are available
• add Python 3.14 and 3.15 to the CI matrix and package classifiers
• add a mypy consumer usage check for typed SDK usage
• add package smoke testing for wheel/sdist contents and installed wheel exports
• add GitHub Actions CI with lint, typecheck, tests, build, twine check, and package smoke checks
• update README to reflect typed request exports and package checks

Verification

• ruff check .
• mypy src tests/typecheck_usage.py
• pytest -> 26 passed, 11 skipped optional live tests
• python -m build
• twine check dist/*
• python scripts/test_pack.py
• GitHub Actions CI passed for Python 3.11, 3.12, 3.13, 3.14, and 3.15