Don't return BUSY_WORKFLOW on polls (#7197)

## What changed?
Follow-up from #7093: exclude `RESOURCE_EXHAUSTED_CAUSE_BUSY_WORKFLOW`
from early return to client.

## Why?
Busy workflow is really a "workflow" scope error, not namespace or
system scope, so the argument that we shouldn't retry immediately
doesn't apply.

## How did you test it?
there weren't specific tests for this

service/matching/matching_engine.go

@@ -652,7 +652,7 @@ pollLoop:
 		}
 		resp, err := e.recordWorkflowTaskStarted(ctx, requestClone, task)
 		if err != nil {
-			switch err.(type) {
+			switch err := err.(type) {
 			case *serviceerror.Internal, *serviceerror.DataLoss:
 				if e.config.MatchingDropNonRetryableTasks() {
 					e.nonRetryableErrorsDropTask(task, taskQueueName, err)
@@ -707,8 +707,11 @@ pollLoop:
 			case *serviceerror.ResourceExhausted:
 				// If history returns one ResourceExhausted, it's likely to return more if we retry
 				// immediately. Instead, return the error to the client which will back off.
+				// BUSY_WORKFLOW is limited to one workflow and is okay to retry.
 				task.finish(err, false)
-				return nil, err
+				if err.Cause != enumspb.RESOURCE_EXHAUSTED_CAUSE_BUSY_WORKFLOW {
+					return nil, err
+				}
 			default:
 				task.finish(err, false)
 				if err.Error() == common.ErrNamespaceHandover.Error() {
@@ -842,7 +845,7 @@ pollLoop:
 		}
 		resp, err := e.recordActivityTaskStarted(ctx, requestClone, task)
 		if err != nil {
-			switch err.(type) {
+			switch err := err.(type) {
 			case *serviceerror.Internal, *serviceerror.DataLoss:
 				if e.config.MatchingDropNonRetryableTasks() {
 					e.nonRetryableErrorsDropTask(task, taskQueueName, err)
@@ -910,8 +913,11 @@ pollLoop:
 			case *serviceerror.ResourceExhausted:
 				// If history returns one ResourceExhausted, it's likely to return more if we retry
 				// immediately. Instead, return the error to the client which will back off.
+				// BUSY_WORKFLOW is limited to one workflow and is okay to retry.
 				task.finish(err, false)
-				return nil, err
+				if err.Cause != enumspb.RESOURCE_EXHAUSTED_CAUSE_BUSY_WORKFLOW {
+					return nil, err
+				}
 			default:
 				task.finish(err, false)
 				if err.Error() == common.ErrNamespaceHandover.Error() {