Allow deferring plain-text MCP output in thread item listings (#60390)

## What changed

Add experimental `deferOutput.mcpToolCall` support to `thread/items/list` requests scoped to a `turnId`. Successful, completed MCP calls with nonempty, unannotated plain-text results can return empty `result.content` with `outputDeferred: true`. Clients can retrieve the saved output through `thread/items/read`; connector identity remains on the listed item.

Keep rich results, JSON or fenced text, app UI results, and built-in `codex_app` results inline. Preserve plain-text output when creating bounded previews of oversized MCP results so those previews remain eligible for deferral. Reads return the saved preview when output exceeds the 64 KiB history limit.

## Testing

Extend thread history integration coverage for independent and combined command/MCP deferral, experimental capability gating, excluded result types, connector identity, oversized previews, and retrieval of deferred items.

GitOrigin-RevId: 5c160b30a4eac2a9b70e3fa65b6dbd83e935338b

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -1879,6 +1879,10 @@ pub struct ThreadItemOutputDeferral {
     #[experimental("thread/items/list.deferOutput.commandExecution")]
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub command_execution: bool,
+    /// Defer nonempty plain-text results from successful MCP tool calls.
+    #[experimental("thread/items/list.deferOutput.mcpToolCall")]
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub mcp_tool_call: bool,
 }
 
 /// Starting position for an item-history page.
@@ -1909,7 +1913,7 @@ pub struct ThreadItemEntry {
     /// Turn containing this item.
     pub turn_id: String,
     pub item: ThreadItem,
-    /// The saved command output was omitted. Fetch the complete item with `thread/items/read`.
+    /// The saved output was omitted. Fetch the complete item with `thread/items/read`.
     #[experimental("thread/items/list.outputDeferred")]
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub output_deferred: bool,