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/core/plugins/built-in/metrics-plugin.ts

@@ -128,14 +128,19 @@ function extractIndexFromPath(path: string): string {
 
   // Split by slash and get first segment
   const segments = cleanPath.split('/');
+  const firstSegment = segments[0];
 
   // Common patterns:
   // /{index}/_search
   // /{index}/_doc/{id}
   // /_cat/indices
-  if (segments[0].startsWith('_')) {
-    return segments[0]; // API endpoint like _cat, _search
+  if (!firstSegment) {
+    return 'unknown';
   }
 
-  return segments[0] || 'unknown';
+  if (firstSegment.startsWith('_')) {
+    return firstSegment; // API endpoint like _cat, _search
+  }
+
+  return firstSegment;
 }