Verify Python 3.9–3.14 compatibility: add classifiers, tests, and CI matrix

[earguinarena]

Motivation

• Declare and verify that the package remains compatible with Python 3.9 through 3.14 while keeping the runtime requirement >=3.9 unchanged.
• Add CI coverage and focused metadata tests to ensure classifiers and requires-python remain accurate across the supported runtimes.

Description

• Added Python 3.13 and 3.14 classifiers to pyproject.toml while retaining requires-python = ">=3.9" and the existing 3.9–3.12 classifiers.
• Introduced tests/test_runtime_compatibility.py which asserts the requires-python entry and that classifiers enumerate exactly 3.9–3.14.
• Added .github/workflows/test.yml with a matrix for Python 3.9, 3.10, 3.11, 3.12, 3.13, and 3.14 where each job installs the package, imports the complete public API, and runs the full test suite.
• Audited production source for Python-3.9-compatible constructs and found no incompatible code changes to runtime behavior, metadata contracts, identity behavior, release workflow, README, or documentation.

Testing

• Ran PYTHONPATH=src pytest -q and PYTHONPATH=src pytest --collect-only -q which resulted in 227 passed and 227 collected, respectively.
• Ran python -m compileall -q src tests successfully and git diff --check showed no issues.
• Verified full test suite under multiple local interpreters with PYENV_VERSION where available and observed 227 passed on Python 3.10, 3.11, 3.12, 3.13, and 3.14, while Python 3.9 was not installed in the local environment.
• Attempted python -m pip install --editable . but the environment could not fetch build dependencies due to a package-index tunnel returning HTTP 403, so tests were executed from src rather than via an editable install; the CI workflow retains an installation step for reproducible runs.

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Codex Task