Add codecov.yaml to properly configure coverage comparison

- Set project target to auto to compare against current base branch
- Set 1% threshold to allow minor coverage fluctuations
- Make project check informational to not block merging
- Configure proper patch coverage targets
- Ignore test files from coverage calculations

.codecov.yaml

@@ -1,11 +1,20 @@
 codecov:
-  # Tell Codecov to compare against upstream master, not commit hash
-  base: "https://github.com/gin-gonic/gin"
-  # Set expected coverage based on current master
   coverage:
     status:
       project:
-        target: 99%
+        # Compare against current base branch, not commit hash
-        informational: true  # Make it informational only
+        target: auto
+        # Only fail if coverage drops significantly (more than 1%)
+        threshold: 1%
+        # Make it informational so it doesn't block merging
+        informational: true
       patch:
         target: 99%
+        threshold: 1%
+    # Set expected coverage
+  comment:
+    layout: "reach, diff, flags, files"
+    behavior: default
+  ignore:
+    - "**/*_test.go"
+    - "**/test_*.go"