Define the top-level public API

[Kkkakania]

Summary

• add a sorted __all__ to hvplot/__init__.py, including __version__ and the existing documented top-level objects
• remove the import-level F401 suppressions that are no longer needed once those imports are declared exports
• add an exact public-API contract test that also verifies every exported name exists

This addresses the top-level portion of #1626.

Context

#1718 previously proposed the same top-level scope and established the intended export set. Thank you to @toroleapinc for that initial work. This replacement starts from current main, applies the maintainer feedback recorded there, adds regression coverage, and includes the requested AI disclosure.

Validation

• regression test verified red on main because hvplot.__all__ was absent
• python -m pytest hvplot/tests/test_api.py hvplot/tests/testdeprecations.py hvplot/tests/testpatchsignature.py hvplot/tests/testplotting.py -q (103 passed, 7 skipped)
• python -m pytest hvplot/tests -q (1039 passed, 376 skipped, 58 deselected, 24 xfailed)
• pre-commit run --files hvplot/__init__.py hvplot/tests/test_api.py (all applicable hooks passed)
• from hvplot import * smoke test
• git diff --check

AI assistance

OpenAI Codex assisted with issue and prior-PR research, implementation, testing, and PR drafting. I reviewed the full diff and validation results. No private data or generated project assets are included.