chore: added codecov.yml for excluding irrelevant code from codecoverage report

Author: f7f376a1fcd0d0e11a10ed1b6577c9

codecov.yml

@@ -0,0 +1,19 @@
+coverage:
+  status:
+    project:
+      default:
+        target: auto
+        threshold: 2%
+    patch:
+      default:
+        target: auto
+
+ignore:
+  - "testutil/**"       # Test fixtures and helpers
+  - "tests/**"          # Test files (already excluded by Go but explicit)
+  - "dag/types.go"      # Type definitions and trivial setters - tested indirectly
+
+comment:
+  layout: "reach,diff,flags,files"
+  behavior: default
+  require_changes: false