fix: addHeaders default param and mergeAbortSignals order in RequestConfigBuilder (#615)

- Add default empty object parameter to addHeaders() so calling with
  undefined no longer throws TypeError from Object.keys(undefined)
- Reorder mergeAbortSignals to check primarySignal.aborted before
  allocating AbortController, preventing unnecessary controller creation

Author: easonliang28

src/api/providers/config-builder/request-config-builder.ts

@@ -38,7 +38,7 @@ export class RequestConfigBuilder<TOptions extends Record<string, any> = Record<
 	 * @param headers - Key-value pairs of header names and values
 	 * @returns this for chainable calls
 	 */
-	addHeaders(headers: Record<string, string>): this {
+	addHeaders(headers: Record<string, string> = {}): this {
 		if (Object.keys(headers).length === 0) {
 			return this
 		}
@@ -121,12 +121,12 @@ export class RequestConfigBuilder<TOptions extends Record<string, any> = Record<
 			return primarySignal
 		}
 
-		const controller = new AbortController()
-
 		if (primarySignal.aborted) {
 			return primarySignal
 		}
 
+		const controller = new AbortController()
+
 		primarySignal.addEventListener("abort", () => controller.abort(), { once: true })
 		secondarySignal.addEventListener("abort", () => controller.abort(), { once: true })