test(app-server): cover thread turn context updates

Document `thread/turnContext/update` more clearly in the protocol and
README, including that it updates the live defaults used by later
thread-scoped operations.

Add a focused app-server regression test that updates a loaded
thread's cwd and verifies `thread/read` reflects the new value.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -3107,6 +3107,7 @@
       "type": "object"
     },
     "ThreadTurnContextUpdateParams": {
+      "description": "Parameters for `thread/turnContext/update`. Each field is optional; only supplied fields override the thread's defaults for subsequent turns and thread-scoped operations (e.g. shell commands).",
       "properties": {
         "approvalPolicy": {
           "anyOf": [

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -14191,6 +14191,7 @@
       },
       "ThreadTurnContextUpdateParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
+        "description": "Parameters for `thread/turnContext/update`. Each field is optional; only supplied fields override the thread's defaults for subsequent turns and thread-scoped operations (e.g. shell commands).",
         "properties": {
           "approvalPolicy": {
             "anyOf": [

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -12046,6 +12046,7 @@
     },
     "ThreadTurnContextUpdateParams": {
       "$schema": "http://json-schema.org/draft-07/schema#",
+      "description": "Parameters for `thread/turnContext/update`. Each field is optional; only supplied fields override the thread's defaults for subsequent turns and thread-scoped operations (e.g. shell commands).",
       "properties": {
         "approvalPolicy": {
           "anyOf": [

codex-rs/app-server-protocol/schema/json/v2/ThreadTurnContextUpdateParams.json

@@ -336,6 +336,7 @@
       "type": "object"
     }
   },
+  "description": "Parameters for `thread/turnContext/update`. Each field is optional; only supplied fields override the thread's defaults for subsequent turns and thread-scoped operations (e.g. shell commands).",
   "properties": {
     "approvalPolicy": {
       "anyOf": [

codex-rs/app-server-protocol/schema/typescript/v2/ThreadTurnContextUpdateParams.ts

@@ -10,6 +10,11 @@ import type { ApprovalsReviewer } from "./ApprovalsReviewer";
 import type { AskForApproval } from "./AskForApproval";
 import type { SandboxPolicy } from "./SandboxPolicy";
 
+/**
+ * Parameters for `thread/turnContext/update`. Each field is optional; only
+ * supplied fields override the thread's defaults for subsequent turns and
+ * thread-scoped operations (e.g. shell commands).
+ */
 export type ThreadTurnContextUpdateParams = {threadId: string, /**
  * Override the working directory for subsequent turns and thread-scoped
  * operations.

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

@@ -2896,6 +2896,9 @@ pub struct ThreadSetNameResponse {}
 )]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
+/// Parameters for `thread/turnContext/update`. Each field is optional; only
+/// supplied fields override the thread's defaults for subsequent turns and
+/// thread-scoped operations (e.g. shell commands).
 pub struct ThreadTurnContextUpdateParams {
     pub thread_id: String,
     /// Override the working directory for subsequent turns and thread-scoped