Keep external agent detection from blocking config requests (#37528)

## Why

External agent detection can spend time reading session transcripts. Sharing the
configuration serialization queue causes unrelated configuration requests to
wait for detection to finish.

## What changed

- Give `externalAgentConfig/detect` its own global serialization queue.
- Keep configuration reads, requirements reads, writes, and external agent
  imports responsive while detection is still running.

## Testing

Add a Unix app-server test that blocks transcript detection on a FIFO and
verifies the unrelated requests complete before detection is released.

GitOrigin-RevId: d2764dcdb0e96d810c790c570c48fa45690a3dc1

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -1179,7 +1179,7 @@ client_request_definitions! {
     },
     ExternalAgentConfigDetect => "externalAgentConfig/detect" {
         params: v2::ExternalAgentConfigDetectParams,
-        serialization: global("config"),
+        serialization: global("external-agent-detect"),
         response: v2::ExternalAgentConfigDetectResponse,
     },
     ExternalAgentConfigImport => "externalAgentConfig/import" {