ut: add ci guard for ut coverage (#2317)

### What this PR does / why we need it?
add ci guard for ut coverage, if ut coverage of patch pr is below 80%,
the ci will failed/

### Does this PR introduce _any_ user-facing change?
not involved

### How was this patch tested?
not involved

- vLLM version: v0.10.0
- vLLM main:
vllm-project/vllm@458e74e

---------

Signed-off-by: Ronald1995 <[email protected]>

Author: Ronald1995

codecov.yml

@@ -17,12 +17,10 @@
 
 coverage:
   status:
-    # non-voting, new code must be fully tested
+    # Patch coverage is mandatory and must be >= 80%
     patch:
       default:
-        target: 100%
-        # non-voting
-        informational: true
+        target: 80%
     # non-voting
     project:
       default: