Add experimental thread memory summary reads (#56465)

## What changed

Add `thread/memory/read` to return `{summary}` for a thread that has been started or resumed. Read `memory_summary.md` from the thread's resolved memory version, honoring thread-specific configuration overrides without falling back to another version.

Return `null` when memory use is disabled or the selected summary is missing, unreadable, or blank.

## Testing

Extend app-server integration coverage to check explicit and automatic memory version selection, disabled memory use, and missing summaries without fallback.

GitOrigin-RevId: b218f31e379c69ccbacb444bb9d6ee920cd4882b

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

@@ -720,6 +720,12 @@ client_request_definitions! {
         serialization: thread_id(params.thread_id),
         response: v2::ThreadMemoryModeSetResponse,
     },
+    #[experimental("thread/memory/read")]
+    ThreadMemoryRead => "thread/memory/read" {
+        params: v2::ThreadMemoryReadParams,
+        serialization: thread_id(params.thread_id),
+        response: v2::ThreadMemoryReadResponse,
+    },
     #[experimental("memory/status")]
     MemoryStatus => "memory/status" {
         params: v2::MemoryStatusParams,

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

@@ -1,10 +1,27 @@
-//! Readiness for selecting v2 memory context after background consolidation.
+//! Memory readiness and summary reads using a loaded thread's selected pipeline.
 
 use crate::JsonSchema;
 use crate::TS;
 use serde::Deserialize;
 use serde::Serialize;
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadMemoryReadParams {
+    /// The thread must already be started or resumed.
+    pub thread_id: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadMemoryReadResponse {
+    /// Summary from the thread's selected memory version. Null when memory use is
+    /// disabled or the selected summary is missing, unreadable, or blank.
+    pub summary: Option<String>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]