CI does not exercise supported Python 3.13 in test jobs

[krystophny]

The repo advertises Python 3.13 support in pyproject.toml, and CI already uses 3.13 for lint, but the actual test jobs do not exercise 3.13:

• test-matrix only runs on 3.10, 3.11, and 3.12
• the Apple Silicon MLX job is pinned to 3.11

That means supported 3.13 is not covered by the meaningful test jobs, especially the Apple Silicon/MLX path where most runtime behavior lives.

A few observations from local verification on Apple Silicon:

• upstream main full-suite runs do reproduce async-test harness coverage problems when pytest-asyncio is not present in the environment, so 3.13 coverage is meaningful
• upstream main does not currently reproduce the fork-specific tests/test_server.py event-loop failure, so I am not reporting that here
• contributor docs do mention pip install -e "[dev]", but they do not clearly distinguish the pure-Python subset from the full Apple-Silicon MLX suite

Suggested fix:

1. add 3.13 to test-matrix
2. add 3.13 to the Apple Silicon test job as well
3. keep the current Linux/Apple split, since the MLX-dependent tests genuinely require Apple Silicon
4. clarify contributor docs for the two local test paths:
   • pure-Python / CI-like subset
   • full Apple-Silicon MLX suite

This would align CI with the project's stated Python support and make local reproduction clearer for contributors.