Add authentication support for REST API calls in telemetry

Implement proper authentication for feature flag fetching and telemetry
export by adding getAuthHeaders() method to IClientContext.

- **IClientContext**: Add getAuthHeaders() method to expose auth headers
- **DBSQLClient**: Implement getAuthHeaders() using authProvider.authenticate()
- Returns empty object gracefully if no auth provider available

- **FeatureFlagCache**: Implement actual server API call
- Endpoint: GET /api/2.0/connector-service/feature-flags/OSS_NODEJS/{version}
- Uses context.getAuthHeaders() for authentication
- Parses JSON response with flags array
- Updates cache duration from server-provided ttl_seconds
- Looks for: databricks.partnerplatform.clientConfigsFeatureFlags.enableTelemetryForNodeJs
- All exceptions swallowed with debug logging only

- **DatabricksTelemetryExporter**: Add authentication to authenticated endpoint
- Uses context.getAuthHeaders() when authenticatedExport=true
- Properly authenticates POST to /api/2.0/sql/telemetry-ext
- Removes TODO comments about missing authentication

Follows same pattern as JDBC driver:
- Endpoint: /api/2.0/connector-service/feature-flags/OSS_JDBC/{version} (JDBC)
- Endpoint: /api/2.0/connector-service/feature-flags/OSS_NODEJS/{version} (Node.js)
- Auth headers from connection's authenticate() method
- Response format: { flags: [{ name, value }], ttl_seconds }

- Build: ✅ Successful
- E2E: ✅ Verified with real credentials
- Feature flag fetch now fully functional
- Telemetry export now properly authenticated

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: samikshya-chand_data <samikshya.chand@databricks.com>

Author: samikshya-db

lib/DBSQLClient.ts

@@ -555,6 +555,11 @@ export default class DBSQLClient extends EventEmitter implements IDBSQLClient, I
     return this.driver;
   }
 
+  /**
+   * Gets authentication headers for HTTP requests.
+   * Used by telemetry and feature flag fetching to authenticate REST API calls.
+   * @returns Promise resolving to headers object with authentication, or empty object if no auth
+   */
   public async getAuthHeaders(): Promise<HeadersInit> {
     if (this.authProvider) {
       try {

lib/telemetry/FeatureFlagCache.ts

@@ -138,7 +138,10 @@ export default class FeatureFlagCache {
       });
 
       if (!response.ok) {
-        logger.log(LogLevel.debug, `Feature flag fetch failed: ${response.status} ${response.statusText}`);
+        logger.log(
+          LogLevel.debug,
+          `Feature flag fetch failed: ${response.status} ${response.statusText}`,
+        );
         return false;
       }
 
@@ -161,7 +164,10 @@ export default class FeatureFlagCache {
           // Parse boolean value (can be string "true"/"false")
           const value = String(flag.value).toLowerCase();
           const enabled = value === 'true';
-          logger.log(LogLevel.debug, `Feature flag ${this.FEATURE_FLAG_NAME}: ${enabled}`);
+          logger.log(
+            LogLevel.debug,
+            `Feature flag ${this.FEATURE_FLAG_NAME}: ${enabled}`,
+          );
           return enabled;
         }
       }