Add a CI check to prevent coverage regressions in pull requests

[chensuyue]

Summary

Ensure that every pull request does not reduce test coverage.

Proposal

Add a corresponding check in CI to detect coverage regressions and fail the workflow when coverage drops below the baseline introduced by the PR.

Expected outcome

• Prevent coverage regressions from being merged
• Make coverage changes visible during PR validation
• Enforce a consistent quality bar in CI

[wenhuach21]

Please do not add this to the CI checks, as it would significantly affect developer productivity. I don't see similar checkers in SGLang, vLLM, or Transformers. I think it would be better to use this as reference data instead.

[chensuyue]

It's hard to guarantee the code quality if there are no test cover the new feature. And there are always coverage regression and need someone to do an extend coverage enhancement for a while.
I will give a warning comments if there are regression instead of break CI. If PR owner could not close the comments in this PR, it can be converted to an issue.

[wenhuach21]

A simple approach is to ask each team member to manually quantize a new model every month, especially a large model, just like I did. This would give them a strong incentive to hate bugs and regressions, catch them early, and add UTs to prevent them. Just a suggestion~