fix(amazonq): add throttling exception handling and in code comments for featureDev quota (#6093)

## Problem
- Follow up of #6091 
- improve and refactor the code from that previous PR

## Solution

Author: willyyhuang

packages/core/src/amazonqFeatureDev/client/featureDev.ts

@@ -78,7 +78,11 @@ export class FeatureDevClient {
                 getLogger().error(
                     `${featureName}: failed to start conversation: ${e.message} RequestId: ${e.requestId}`
                 )
-                if (e.code === 'ServiceQuotaExceededException') {
+                // BE service will throw ServiceQuota if conversation limit is reached. API Front-end will throw Throttling with this message if conversation limit is reached
+                if (
+                    e.code === 'ServiceQuotaExceededException' ||
+                    (e.code === 'ThrottlingException' && e.message.includes('reached for this month.'))
+                ) {
                     throw new MonthlyConversationLimitError(e.message)
                 }
                 throw new ApiError(e.message, 'CreateConversation', e.code, e.statusCode ?? 400)
@@ -167,14 +171,22 @@ export class FeatureDevClient {
                     (e as any).requestId
                 }`
             )
-            if (
-                isAwsError(e) &&
-                ((e.code === 'ThrottlingException' &&
-                    e.message.includes('limit for number of iterations on a code generation')) ||
-                    e.message.includes('StartTaskAssistCodeGeneration reached for this month.') ||
-                    e.code === 'ServiceQuotaExceededException')
-            ) {
-                throw new CodeIterationLimitError()
+            if (isAwsError(e)) {
+                // API Front-end will throw Throttling if conversation limit is reached. API Front-end monitors StartCodeGeneration for throttling
+                if (
+                    e.code === 'ThrottlingException' &&
+                    e.message.includes('StartTaskAssistCodeGeneration reached for this month.')
+                ) {
+                    throw new MonthlyConversationLimitError(e.message)
+                }
+                // BE service will throw ServiceQuota if code generation iteration limit is reached
+                else if (
+                    e.code === 'ServiceQuotaExceededException' ||
+                    (e.code === 'ThrottlingException' &&
+                        e.message.includes('limit for number of iterations on a code generation'))
+                ) {
+                    throw new CodeIterationLimitError()
+                }
             }
             throw new ToolkitError((e as Error).message, { code: 'StartCodeGenerationFailed' })
         }