Extend selective testing with implicit consumers (#268256)

## Summary

Some plugins use registries to inject functionality. Both our strategies
for detecting affected modules cannot discover it, because they rely on
static dependencies. We need a better way to handle it.

While we discuss a general solution, this PR serves as a quick fix that
validates potential implicit consumers on top of computed affected
packages and extends "affected modules" if code changes match any of
these patterns:

```
      '**/plugins/**/public/embeddables/**/*.{ts,tsx}',
      '**/plugins/**/public/embeddable/**/*.{ts,tsx}',
      '**/plugins/**/public/react_embeddable/**/*.{ts,tsx}',
      '**/plugins/**/public/apps/embeddables/**/*.{ts,tsx}',
      '**/plugins/**/public/ui_actions/**/*.{ts,tsx}',
      '**/plugins/**/public/trigger_actions/**/*.{ts,tsx}',
      '**/plugins/**/public/**/actions/register*.{ts,tsx}',
```

Example:

Changing
`x-pack/platform/plugins/shared/ml/public/embeddables/anomaly_charts/anomaly_charts_embeddable_factory.tsx`
should trigger Scout tests in `dashboard`, `lens`, `canvas` and
`embeddable` plugins

<img width="1954" height="279" alt="Screenshot 2026-05-07 at 19 10 42"
src="https://github.com/user-attachments/assets/6c0b4163-481b-4f34-84b2-290ab3091d96"
/>

(cherry picked from commit 86477a0)

Author: dmlemeshko

.buildkite/scripts/steps/test/scout/resolve_selective_testing.ts

@@ -9,13 +9,17 @@
 
 import fs from 'node:fs';
 import path from 'node:path';
+import { ToolingLog } from '@kbn/tooling-log';
+import { expandWithImplicitConsumers } from './scout_implicit_consumers';
 import {
   getAffectedPackages,
   listChangedFiles,
   touchedCriticalFiles,
   CRITICAL_FILES_SCOUT,
 } from '#pipeline-utils';
 
+const log = new ToolingLog({ level: 'info', writeTo: process.stderr });
+
 const mergeBase = process.env.AFFECTED_MERGE_BASE;
 const outPath = process.env.AFFECTED_MODULES_FILE;
 
@@ -34,11 +38,17 @@ if (!outPath) {
   const changedFiles = listChangedFiles({ mergeBase, commit: 'HEAD' });
 
   // Write affected modules JSON (replaces the list_affected binary call).
-  const affectedPackages = await getAffectedPackages(mergeBase, {
-    strategy: 'git',
-    includeDownstream: true,
-    ignoreUncategorizedChanges: true,
-  });
+  // TEMP: overlay implicit runtime-registry consumers — see scout_implicit_consumers.ts.
+  const affectedPackages = expandWithImplicitConsumers(
+    await getAffectedPackages(mergeBase, {
+      strategy: 'git',
+      includeDownstream: true,
+      ignoreUncategorizedChanges: true,
+    }),
+    changedFiles,
+    log
+  );
+
   const resolvedOutPath = path.resolve(outPath);
   fs.mkdirSync(path.dirname(resolvedOutPath), { recursive: true });
   fs.writeFileSync(resolvedOutPath, JSON.stringify(Array.from(affectedPackages).sort(), null, 2));

.buildkite/scripts/steps/test/scout/scout_implicit_consumers.ts

@@ -0,0 +1,82 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+/**
+ * TEMPORARY overlay for Scout selective testing.
+ *
+ * The static @kbn/ dependency graph (kibana.jsonc + tsconfig kbn_references)
+ * cannot model runtime registry coupling — e.g. ML registers actions into the
+ * uiActions registry that Dashboard renders at run time, with no static import
+ * edge between them. ML-only changes therefore do not mark Dashboard's Scout
+ * tests as affected and registration races slip through.
+ *
+ * This overlay augments the affected-modules set with a small allowlist of
+ * (patterns -> consumer @kbn/ IDs) entries when the corresponding publisher
+ * files change.
+ */
+
+import minimatch from 'minimatch';
+import type { ToolingLog } from '@kbn/tooling-log';
+
+interface ImplicitConsumerRule {
+  reason: string;
+  patterns: readonly string[];
+  consumers: readonly string[];
+}
+
+const IMPLICIT_REGISTRY_CONSUMERS: readonly ImplicitConsumerRule[] = [
+  {
+    reason: 'Runtime registry coupling not captured by static @kbn/ references.',
+    patterns: [
+      '**/plugins/**/public/embeddables/**/*.{ts,tsx}',
+      '**/plugins/**/public/embeddable/**/*.{ts,tsx}',
+      '**/plugins/**/public/react_embeddable/**/*.{ts,tsx}',
+      '**/plugins/**/public/apps/embeddables/**/*.{ts,tsx}',
+      '**/plugins/**/public/ui_actions/**/*.{ts,tsx}',
+      '**/plugins/**/public/trigger_actions/**/*.{ts,tsx}',
+      '**/plugins/**/public/**/actions/register*.{ts,tsx}',
+    ],
+    consumers: [
+      '@kbn/dashboard-plugin',
+      '@kbn/embeddable-plugin',
+      '@kbn/canvas-plugin',
+      '@kbn/lens-plugin',
+    ],
+  },
+];
+
+/**
+ * Augment an affected-modules set with consumer @kbn/ IDs whose registries are
+ * touched by `changedFiles`. Returns a new Set; never removes entries and
+ * never disables selective testing.
+ */
+export function expandWithImplicitConsumers(
+  affected: ReadonlySet<string>,
+  changedFiles: readonly string[],
+  log: ToolingLog
+): Set<string> {
+  const expanded = new Set(affected);
+
+  for (const rule of IMPLICIT_REGISTRY_CONSUMERS) {
+    const trigger = changedFiles.find((file) =>
+      rule.patterns.some((pattern) => minimatch(file, pattern, { dot: true }))
+    );
+    if (!trigger) continue;
+
+    const added = rule.consumers.filter((id) => !expanded.has(id));
+    if (added.length === 0) continue;
+
+    for (const id of added) expanded.add(id);
+    log.info(
+      `Implicit consumers added: ${added.join(', ')} (triggered by '${trigger}' — ${rule.reason})`
+    );
+  }
+
+  return expanded;
+}