fix(core): Resolve TypeScript strict mode and ES client API compatibility issues for v3.0.0

- Fix ES client v8+ API: use document/doc instead of body for index/update operations
- Add type assertions (as any) for ES client ILM, template, and search APIs
- Fix strict null checks with proper undefined handling (nullish coalescing)
- Fix MetricsCollector interface to match required method signatures
- Fix Logger.error signature compatibility in plugins
- Resolve TermsQuery type index signature conflict
- Remove sourceMap from tsconfig (handled by tsbuild with inlineSourceMap)

ts/domain/query/aggregation-builder.ts

@@ -274,12 +274,16 @@ export class AggregationBuilder {
   /**
    * Add a sub-aggregation to the last defined aggregation
    */
-  subAggregation(name: string, configure: (builder: AggregationBuilder) => void): this {
+  subAggregation(_name: string, configure: (builder: AggregationBuilder) => void): this {
     if (!this.currentAggName) {
       throw new Error('Cannot add sub-aggregation: no parent aggregation defined');
     }
 
     const parentAgg = this.aggregations[this.currentAggName];
+    if (!parentAgg) {
+      throw new Error('Parent aggregation not found');
+    }
+
     const subBuilder = new AggregationBuilder();
     configure(subBuilder);