[Synthetics] Fix monitor status rule for empty kql query results !! (#208922)

## Summary

Fixes https://github.com/elastic/kibana/issues/208915 !!

Fix monitor status rule for empty kql query results !!

1. Made sure if kql filter return no configs ids, rule break early to
not cover all monitors

### Testing

Create a synthetics rule with a kql filter which matches no monitors and
make sure rule doesn't trigger for other down monitors in the system
<img width="661" alt="image"
src="https://github.com/user-attachments/assets/ed0b3a1f-e8d1-4e22-a77d-1237ce557ac5"
/>


### Before

Create a rule and you can observe that rule would get triggered for all
monitors down in the system with matching condition criteria

x-pack/solutions/observability/plugins/synthetics/server/alert_rules/status_rule/queries/filter_monitors.test.ts

@@ -0,0 +1,80 @@
+/*
+ * 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; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { getFilters } from './filter_monitors';
+
+describe('getFilters', () => {
+  it('returns an empty array when all parameters are empty', () => {
+    const ruleParams = { monitorTypes: [], locations: [], tags: [], projects: [] };
+    expect(getFilters(ruleParams)).toEqual([]);
+  });
+
+  it('creates a terms filter for monitorTypes', () => {
+    const ruleParams = { monitorTypes: ['http', 'tcp'], locations: [], tags: [], projects: [] };
+    expect(getFilters(ruleParams)).toEqual([{ terms: { 'monitor.type': ['http', 'tcp'] } }]);
+  });
+
+  it('creates a terms filter for locations', () => {
+    const ruleParams = {
+      monitorTypes: [],
+      locations: ['us-east', 'us-west'],
+      tags: [],
+      projects: [],
+    };
+    expect(getFilters(ruleParams)).toEqual([
+      { terms: { 'observer.name': ['us-east', 'us-west'] } },
+    ]);
+  });
+
+  it('creates a bool must filter for tags', () => {
+    const ruleParams = { monitorTypes: [], locations: [], tags: ['tag1', 'tag2'], projects: [] };
+    expect(getFilters(ruleParams)).toEqual([
+      {
+        terms: { tags: ['tag1', 'tag2'] },
+      },
+    ]);
+  });
+
+  it('creates a terms filter for projects', () => {
+    const ruleParams = { monitorTypes: [], locations: [], tags: [], projects: ['proj1', 'proj2'] };
+    expect(getFilters(ruleParams)).toEqual([
+      { terms: { 'monitor.project.id': ['proj1', 'proj2'] } },
+    ]);
+  });
+
+  it('handles all filters together', () => {
+    const ruleParams = {
+      monitorTypes: ['http'],
+      locations: ['us-east'],
+      tags: ['tag1', 'tag2'],
+      projects: ['proj1'],
+    };
+    expect(getFilters(ruleParams)).toEqual([
+      { terms: { 'monitor.type': ['http'] } },
+      { terms: { 'observer.name': ['us-east'] } },
+      {
+        terms: { tags: ['tag1', 'tag2'] },
+      },
+      { terms: { 'monitor.project.id': ['proj1'] } },
+    ]);
+  });
+
+  it('ignores undefined fields', () => {
+    const ruleParams = {
+      monitorTypes: undefined,
+      locations: undefined,
+      tags: undefined,
+      projects: undefined,
+    };
+    expect(getFilters(ruleParams)).toEqual([]);
+  });
+
+  it('ignores empty values in an otherwise valid object', () => {
+    const ruleParams = { monitorTypes: [], locations: ['us-east'], tags: [], projects: [] };
+    expect(getFilters(ruleParams)).toEqual([{ terms: { 'observer.name': ['us-east'] } }]);
+  });
+});

x-pack/solutions/observability/plugins/synthetics/server/alert_rules/status_rule/queries/filter_monitors.ts

@@ -65,7 +65,7 @@ export async function queryFilterMonitors({
   return result.aggregations?.ids.buckets.map((bucket) => bucket.key as string);
 }
 
-const getFilters = (ruleParams: StatusRuleParams) => {
+export const getFilters = (ruleParams: StatusRuleParams) => {
   const { monitorTypes, locations, tags, projects } = ruleParams;
   const filters: QueryDslQueryContainer[] = [];
   if (monitorTypes?.length) {

x-pack/solutions/observability/plugins/synthetics/server/alert_rules/status_rule/status_rule_executor.ts

@@ -110,6 +110,11 @@ export class StatusRuleExecutor {
       ruleParams: this.params,
     });
 
+    if (this.params.kqlQuery && isEmpty(configIds)) {
+      this.debug(`No monitor found with the given KQL query ${this.params.kqlQuery}`);
+      return processMonitors([]);
+    }
+
     const { filtersStr } = parseArrayFilters({
       configIds,
       filter: baseFilter,