[CI] Add ut coverage tool (#20628)

.coveragerc

@@ -0,0 +1,16 @@
+[run]
+source = python/sglang/srt
+omit =
+    */test/*
+    */__pycache__/*
+
+[report]
+show_missing = true
+exclude_lines =
+    pragma: no cover
+    if __name__ == .__main__.:
+    raise NotImplementedError
+    if TYPE_CHECKING
+
+[html]
+directory = htmlcov

python/pyproject.toml

@@ -141,6 +141,8 @@ test = [
   "parameterized",
   "peft",
   "pytest",
+  "pytest-cov",
+  "diff-cover",
   "sentence_transformers",
   "tabulate",
 ]

test/registered/unit/README.md

@@ -22,6 +22,15 @@ Tests can use CPU or GPU — the key criterion is **no server process**.
    pytest test/registered/unit/ -v            # all unit tests
    pytest test/registered/unit/mem_cache/ -v  # one module
    ```
+5. Run with coverage:
+   ```bash
+   # summary
+   pytest test/registered/unit/ --cov --cov-config=.coveragerc -v
+
+   # PR incremental check (require ≥60% on changed lines)
+   pytest test/registered/unit/ --cov --cov-config=.coveragerc --cov-report=xml
+   diff-cover coverage.xml --compare-branch=origin/main --fail-under=60
+   ```
 
 ## Example