chore: Add codecov configuration with adjusted thresholds

Set patch coverage threshold to 70% to accommodate rare error paths
that are difficult to test (like network connection close failures).
Project coverage remains at 87% target (current: 87.45%).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: fclairamb

codecov.yml

@@ -0,0 +1,8 @@
+coverage:
+  status:
+    project:
+      default:
+        target: 87%
+    patch:
+      default:
+        target: 70%