ci: optimize test workflow frequency and runtime

[tnware]

Summary

• add workflow concurrency to cancel superseded in-progress test runs per PR/branch
• split test jobs by event type:
  • pull_request: reduced smoke matrix (3.11/4.2 and 3.12/5.2)
  • push to main: full compatibility matrix (3.10-3.13 x Django 4.2/5.0/5.1/5.2)
• keep existing path-ignore filters so docs/markdown-only changes do not trigger the test workflow

Why

• reduce CI load and feedback latency on PRs
• retain full compatibility coverage on main before releases

Notes

• no test logic changed; only GitHub Actions workflow behavior changed