feat: dynamic tools injection (#9539)

## Summary
Add dynamic tool injection to thread startup in API v2, wire dynamic
tool calls through the app server to clients, and plumb responses back
into the model tool pipeline.

### Flow (high level)
- Thread start injects `dynamic_tools` into the model tool list for that
thread (validation is done here).
- When the model emits a tool call for one of those names, core raises a
`DynamicToolCallRequest` event.
- The app server forwards it to the client as `item/tool/call`, waits
for the client’s response, then submits a `DynamicToolResponse` back to
core.
- Core turns that into a `function_call_output` in the next model
request so the model can continue.

### What changed
- Added dynamic tool specs to v2 thread start params and protocol types;
introduced `item/tool/call` (request/response) for dynamic tool
execution.
- Core now registers dynamic tool specs at request time and routes those
calls via a new dynamic tool handler.
- App server validates tool names/schemas, forwards dynamic tool call
requests to clients, and publishes tool outputs back into the session.
- Integration tests

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

@@ -325,6 +325,15 @@ pub struct ToolsV2 {
     pub view_image: Option<bool>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct DynamicToolSpec {
+    pub name: String,
+    pub description: String,
+    pub input_schema: JsonValue,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "snake_case")]
 #[ts(export_to = "v2/")]
@@ -1090,6 +1099,7 @@ pub struct ThreadStartParams {
     pub developer_instructions: Option<String>,
     pub personality: Option<Personality>,
     pub ephemeral: Option<bool>,
+    pub dynamic_tools: Option<Vec<DynamicToolSpec>>,
     /// If true, opt into emitting raw response items on the event stream.
     ///
     /// This is for internal use only (e.g. Codex Cloud).
@@ -2374,6 +2384,25 @@ pub struct FileChangeRequestApprovalResponse {
     pub decision: FileChangeApprovalDecision,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct DynamicToolCallParams {
+    pub thread_id: String,
+    pub turn_id: String,
+    pub call_id: String,
+    pub tool: String,
+    pub arguments: JsonValue,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct DynamicToolCallResponse {
+    pub output: String,
+    pub success: bool,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]