Pre-release validation fixes and test coverage

[haoyuhu]

Summary:

• Fix Python 3.9 runtime compatibility for type annotations.
• Harden error parsing for non-JSON HTTP/SSE responses.
• Fix packaging to include subpackages.
• Align package constraints (python>=3.8, pydantic>=2,<3).
• Modernize build script and CI checks.
• Add unit tests for client behavior, error mapping, and model defaults.

Validation:

• python3 -m pytest -q
• python3 -m flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
• python3 -m build --no-isolation
• python3 -m twine check --strict dist/*