Use libclang package from PyPI

[jbcoe]

closes #11

[Copilot]

Pull request overview

This PR modernizes the libclang dependency management by using the libclang package from PyPI instead of requiring users to manually configure the library path via environment variables.

Changes:

• Added libclang>=18.1.1 as a project dependency in pyproject.toml and uv.lock
• Removed manual libclang path configuration code from test files and Jupyter notebook
• Removed the PY_CPPMODEL_LIBCLANG_PATH environment variable requirement from test scripts and CI workflows
• Updated README to remove the "Limitations" section that documented the environment variable requirement

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
uv.lock	Added libclang 18.1.1 package with platform-specific wheels and updated package metadata
test_py_cppmodel.py	Removed environment variable check and manual library configuration
test_parse_standard_library_includes.py	Removed environment variable check and manual library configuration
test.macos.sh	Removed PY_CPPMODEL_LIBCLANG_PATH environment variable from test execution
pyproject.toml	Added libclang>=18.1.1 to project dependencies
Sandbox.ipynb	Removed manual libclang configuration cell and renumbered subsequent cells
README.md	Removed "Limitations" section and updated test instructions
.github/workflows/python.yml	Removed PY_CPPMODEL_LIBCLANG_PATH environment variable from CI test execution

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