fix coverage check config

Author: devalbo

vitest.coverage-check.config.ts

@@ -1,22 +1,22 @@
-import { defineConfig } from 'vitest/config';
-import base from './vitest.config';
+import { defineConfig, mergeConfig } from 'vitest/config';
+import baseConfig from './vitest.config';
 
 // CI/quality gate: enforce minimum coverage thresholds (global).
-export default defineConfig({
-  ...(base as unknown as Record<string, unknown>),
-  test: {
-    // @ts-expect-error - base config shape is compatible at runtime
-    ...(base.test ?? {}),
-    coverage: {
-      // @ts-expect-error - base config shape is compatible at runtime
-      ...(base.test?.coverage ?? {}),
-      thresholds: {
-        lines: 20,
-        functions: 20,
-        branches: 20,
-        statements: 20,
+export default mergeConfig(
+  baseConfig,
+  defineConfig({
+    test: {
+      coverage: {
+        // Re-declare provider to keep types narrow.
+        provider: 'v8',
+        thresholds: {
+          lines: 20,
+          functions: 20,
+          branches: 20,
+          statements: 20,
+        },
       },
     },
-  },
-});
+  })
+);