Add official Python 3.14 support (3.10–3.14 inclusive)

[dgenio]

Summary

Please make ChainWeaver officially support Python versions 3.10 through 3.14 inclusive.

Why

Current metadata and CI indicate support through 3.13, but not 3.14 yet:

• pyproject.toml classifiers list 3.10, 3.11, 3.12, 3.13
• .github/workflows/ci.yml test matrix currently runs 3.10–3.13

Proposed scope

• Add Python 3.14 to package classifiers in pyproject.toml
• Expand CI matrix to include Python 3.14
• Fix any compatibility issues found under 3.14
• Ensure docs/README and any support matrix mention 3.10–3.14 explicitly (if needed)

Acceptance criteria

• CI is green on Python 3.10, 3.11, 3.12, 3.13, and 3.14
• PyPI metadata/classifiers include Python 3.14
• No regressions for existing supported versions

Notes

This issue is specifically to keep support inclusive from 3.10 up to 3.14.