fix(config-builder): fix broken TOC link and simplify mergeAbortSignals early-abort

- Fix README TOC: change #how-mergesignals-works to
  #how-mergeabortsignals-works to match the actual heading anchor
- Simplify mergeAbortSignals: return primarySignal directly when it's
  already aborted instead of creating a new AbortController

Author: easonliang28

src/api/providers/config-builder/README.md

@@ -15,7 +15,7 @@ A generic, SDK-agnostic request configuration builder that provides a fluent API
 - [Deep Dive: Abort Signal Handling](#deep-dive-abort-signal-handling)
     - [Why Abort Signals Matter](#why-abort-signals-matter)
     - [How `addAbortSignal` Works](#how-addabortsignal-works)
-    - [How `mergeAbortSignals` Works](#how-mergesignals-works)
+    - [How `mergeAbortSignals` Works](#how-mergeabortsignals-works)
 - [Generic Design](#generic-design)
 - [Multi-SDK Usage Examples](#multi-sdk-usage-examples)
 - [Extending for Your SDK](#extending-for-your-sdk)

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

@@ -124,8 +124,7 @@ export class RequestConfigBuilder<TOptions extends Record<string, any> = Record<
 		const controller = new AbortController()
 
 		if (primarySignal.aborted) {
-			controller.abort()
-			return controller.signal
+			return primarySignal
 		}
 
 		primarySignal.addEventListener("abort", () => controller.abort(), { once: true })