ci: add Python 3.15 support

[jrepp]

Summary

Add Python 3.15 to Docuchango's supported interpreter range and CI matrix while keeping Python 3.10 as the minimum supported version.

Related Docs

• Updates docs-cms PRD testing support statement in docs-cms/prd/prd-001-validation-framework.md

Changes

• Expand package metadata to support Python >=3.10,<3.16 and add the Python 3.15 classifier.
• Add Python 3.15 to the CI test matrix and move Codecov upload to the newest matrix job.
• Regenerate uv.lock for the expanded interpreter range.
• Keep release/static asset jobs on Python 3.14 and update docs accordingly.
• Add a narrow Pydantic typing compatibility shim for newer Python annotation internals.

Testing

• uv run ruff format --check .
• uv run ruff check .
• uv run mypy docuchango tests
• uv run docuchango validate
• uv run pytest

Note: uv did not expose a local Python 3.15 prerelease download, so local verification ran under the repo-pinned Python 3.14.0rc2. The new CI matrix entry will verify Python 3.15.