fix: Harden mutation picker against low-value source files (#31914)

Co-authored-by: n8n-cat-bot[bot] <n8n-cat-bot[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

scripts/mutation-health/pick-next.mjs

@@ -38,7 +38,7 @@
  */
 
 import { readdir, readFile } from 'node:fs/promises';
-import { existsSync } from 'node:fs';
+import { existsSync, readFileSync } from 'node:fs';
 import { execFileSync } from 'node:child_process';
 import path from 'node:path';
 
@@ -65,20 +65,54 @@ function parseArgs(argv) {
 }
 
 // Files with no useful mutation surface: barrels, declarations, type-only modules.
-const LOW_VALUE_BASENAMES = new Set(['interfaces', 'index', 'constants', 'types']);
+// Matched against either the full basename (`types.ts`) or the trailing
+// dot-segment (`foo.types.ts` → `types`), so dotted-suffix declaration files
+// are caught the same as their plain-named counterparts. `methods` covers
+// NativeDoc `*.methods.ts` descriptor files; `schemas` covers pure Zod
+// declaration files; `message-event-bus` is an exact-basename entry for a
+// bulk enum + interfaces + Zod module with no function logic.
+const LOW_VALUE_BASENAMES = new Set([
+	'interfaces',
+	'index',
+	'constants',
+	'types',
+	'methods',
+	'schemas',
+	'message-event-bus',
+]);
+
+// Files with fewer than this many non-blank, non-import lines have so little
+// surface area they're not worth mutating — e.g. trivial error subclasses with
+// empty bodies or one hardcoded super() call. This catches what an exact-name
+// list can't (`error` can't be skipped wholesale because files like
+// `workflow-activation.error.ts` have real branching logic).
+const MIN_MEANINGFUL_LINES = 15;
 
 // Directories whose contents are tests/fixtures/mocks, not production code.
 // Pruned in `walkSources` so we don't descend; `isMutationWorthy` re-checks as a
 // safety net for packages that co-locate tests as siblings (e.g. `foo.test.ts`).
 const NON_SOURCE_DIRS = new Set(['__tests__', '__mocks__', 'fixtures']);
 
+function countMeaningfulLines(content) {
+	let count = 0;
+	for (const raw of content.split('\n')) {
+		const line = raw.trim();
+		if (!line) continue;
+		if (line.startsWith('import ')) continue;
+		count++;
+	}
+	return count;
+}
+
 function isMutationWorthy(absPath) {
 	if (absPath.endsWith('.d.ts')) return false;
 	if (/\.(test|spec)\.ts$/.test(absPath)) return false;
 	if (absPath.includes(`${path.sep}__tests__${path.sep}`)) return false;
 	if (absPath.includes(`${path.sep}__mocks__${path.sep}`)) return false;
 	const base = path.basename(absPath, '.ts');
-	if (LOW_VALUE_BASENAMES.has(base)) return false;
+	const lastSegment = base.split('.').at(-1);
+	if (LOW_VALUE_BASENAMES.has(base) || LOW_VALUE_BASENAMES.has(lastSegment)) return false;
+	if (countMeaningfulLines(readFileSync(absPath, 'utf8')) < MIN_MEANINGFUL_LINES) return false;
 	return true;
 }