[codex] allow shared config reads in app-server queue (#21340)

## Summary
- add a shared-read serialization mode for global app-server request
families
- let consecutive leading shared reads for the same family run together
while keeping exclusive requests ordered
- mark only `skills/list`, `config/read` and `plugin/list` as shared
reads for now

## Why
`skills/list` and `plugin/list` are read-only config-family requests,
but the app-server queue currently treats every config request as
exclusive. That means one long `skills/list` can make a later
`plugin/list` wait even though the two requests do not mutate config.

This change keeps the existing queue order but lets adjacent reads
overlap. If a write is already waiting, later reads still stay behind
it, so writes do not starve.

## Scope
This intentionally keeps the first pass narrow:
- shared reads: `skills/list`, `plugin/list`
- still exclusive: `plugin/install`, `marketplace/*`,
`skills/config/write`, `config/*write`, `config/read`, and the rest of
the config family

## Validation
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`

## Desktop verification
I ran the dev desktop app against this branch's built binary with the
existing UI timing logs enabled. The app did use
`/Users/xli/code/codex_6/codex-rs/target/debug/codex`.

The new scheduler behavior works, but this narrow change does not remove
every cold-start delay: in the observed trace, an earlier exclusive
`config/read` was already queued ahead of the later `skills/list` and
`plugin/list` requests, so the page-open plugin requests still waited
behind that earlier exclusive config-family request before they could
run together.

That means this PR is the scheduler primitive needed for shared reads,
not the complete end-to-end latency fix by itself.

## Not run
- full workspace test suite, because repo policy requires explicit
approval before running it after touching `app-server-protocol`

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

@@ -77,6 +77,7 @@ macro_rules! experimental_type_entry {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ClientRequestSerializationScope {
     Global(&'static str),
+    GlobalSharedRead(&'static str),
     Thread { thread_id: String },
     ThreadPath { path: PathBuf },
     CommandExecProcess { process_id: String },
@@ -93,6 +94,9 @@ macro_rules! serialization_scope_expr {
     ($actual_params:ident, global($key:literal)) => {
         Some(ClientRequestSerializationScope::Global($key))
     };
+    ($actual_params:ident, global_shared_read($key:literal)) => {
+        Some(ClientRequestSerializationScope::GlobalSharedRead($key))
+    };
     ($actual_params:ident, thread_id($params:ident . $field:ident)) => {
         Some(ClientRequestSerializationScope::Thread {
             thread_id: $actual_params.$field.clone(),
@@ -585,7 +589,7 @@ client_request_definitions! {
     },
     SkillsList => "skills/list" {
         params: v2::SkillsListParams,
-        serialization: global("config"),
+        serialization: global_shared_read("config"),
         response: v2::SkillsListResponse,
     },
     HooksList => "hooks/list" {
@@ -610,7 +614,7 @@ client_request_definitions! {
     },
     PluginList => "plugin/list" {
         params: v2::PluginListParams,
-        serialization: global("config"),
+        serialization: global_shared_read("config"),
         response: v2::PluginListResponse,
     },
     PluginRead => "plugin/read" {
@@ -947,7 +951,7 @@ client_request_definitions! {
 
     ConfigRead => "config/read" {
         params: v2::ConfigReadParams,
-        serialization: global("config"),
+        serialization: global_shared_read("config"),
         response: v2::ConfigReadResponse,
     },
     ExternalAgentConfigDetect => "externalAgentConfig/detect" {
@@ -1655,6 +1659,28 @@ mod tests {
             Some(ClientRequestSerializationScope::Global("config"))
         );
 
+        let skills_list = ClientRequest::SkillsList {
+            request_id: request_id(),
+            params: v2::SkillsListParams {
+                cwds: Vec::new(),
+                force_reload: false,
+                per_cwd_extra_user_roots: None,
+            },
+        };
+        assert_eq!(
+            skills_list.serialization_scope(),
+            Some(ClientRequestSerializationScope::GlobalSharedRead("config"))
+        );
+
+        let plugin_list = ClientRequest::PluginList {
+            request_id: request_id(),
+            params: v2::PluginListParams { cwds: None },
+        };
+        assert_eq!(
+            plugin_list.serialization_scope(),
+            Some(ClientRequestSerializationScope::GlobalSharedRead("config"))
+        );
+
         let plugin_uninstall = ClientRequest::PluginUninstall {
             request_id: request_id(),
             params: v2::PluginUninstallParams {
@@ -1705,7 +1731,7 @@ mod tests {
         };
         assert_eq!(
             config_read.serialization_scope(),
-            Some(ClientRequestSerializationScope::Global("config"))
+            Some(ClientRequestSerializationScope::GlobalSharedRead("config"))
         );
 
         let account_read = ClientRequest::GetAccount {