Only test on Python 3.10 in CI

[MatthewMckee4]

Summary

• Remove the python-version matrix from the cargo-test job, hardcoding to Python 3.10
• Remove python-version from the job name
• Halves the CI matrix from 6 jobs (3 OS × 2 Python) to 3 jobs (3 OS × 1 Python)

Closes #407

Test plan

• Verify CI runs successfully with only Python 3.10
• Confirm cargo-test jobs show as cargo test (ubuntu-latest), cargo test (windows-latest), cargo test (macos-latest)

🤖 Generated with Claude Code

[github-actions]

Project Diff Results

No changes between this PR and main.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing only-test-python-3.10 (de6c5b2) with main (8a799e7)}

Summary

✅ 1 untouched benchmark