Add public API docstrings and error-handling clarity (step 17)

[whitewalls86]

Summary

• Client docstrings — added class and method docstrings to all public methods on AsyncVisorClient and VisorClient, covering one-page vs. paginate semantics, return types, important params, and the full raised-exception list. VisorClient class docstring notes the thread-safety caveat and context-manager usage.
• Exception docstrings — expanded AuthError (401 = missing/invalid key), ForbiddenError (403 = valid key, no permission for resource), and RateLimitError (retry_after attribute: seconds or None, usage example).
• 401/403 fallback messages — when the API returns a 401 or 403 with an empty body, message now falls back to a short actionable default instead of an empty string. API-provided messages are always preferred (api_message or fallback).
• Tests — four new parametrized sync/async tests covering the empty-body fallback path.

Fixes in review

• Removed incorrect paginate_listings/iter_listings with dealer_id hint (those helpers don't accept that parameter).
• Corrected filter.page → filter.offset in dealer_inventory docstrings (SDK uses offset-based pagination).

Test plan

• pytest --cov=visor --cov-report=term-missing — 232 passed, 99% coverage
• ruff check src/ tests/ — no issues
• ruff format --check src/ tests/ — no issues
• mypy src/ — no issues
• python -m build — wheel and sdist built successfully
• python -m twine check dist/* — both artifacts passed
• visor/py.typed present in wheel
• Excluded paths (Visor API Docs/, Design Docs/, .coverage) absent from sdist

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