Add support for Python 3.13 and fix Codecov configuration

[lanpa]

Description

This PR adds official support for Python 3.13 to the test matrix and package metadata, while also fixing the Codecov configuration.

Changes:

• CI Workflows:
  • Added 3.13 to the test matrix in test-matrix.yml.
  • Fixed dependency resolution in example tests by adding PyPI as an extra index (ensures protobuf and other standard packages are found alongside CPU-only torch wheels).
• Codecov:
  • Updated .codecov.yml to set require_head: no, preventing the bot from waiting indefinitely for coverage reports.
• Package Metadata:
  • Added Python 3.13 classifier to pyproject.toml.
• Documentation:
  • Updated README.md to reflect support up to Python 3.13.
  • Updated .readthedocs.yaml to use Python 3.12 for builds.
• Environment: Updated Makefile and build workflows to use Python 3.13 as the default modern version.
• Dependencies: Pinned torch==2.10.0 and torchvision==0.25.0 to ensure stability on newer Python versions.

Verification Results:

• ✅ Local Verification: All 94 tests passed using uv run pytest on Python 3.10.
• ✅ CI Verification: Python 3.10 through 3.13 are now verified in CI, including example tests and Codecov reporting.