Refactor OTEL logging for API calls (#991)

Refactor OpenTelemetry logging for API requests, responses, and errors. Moved logging responsibility from GeminiClient to GeminiChat for more detailed logging.

#750

packages/core/src/utils/requestUtils.ts

@@ -0,0 +1,15 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { Content } from '@google/genai';
+
+export function getRequestTextFromContents(contents: Content[]): string {
+  return contents
+    .flatMap((content) => content.parts ?? [])
+    .map((part) => part.text)
+    .filter(Boolean)
+    .join('');
+}