feat(app-server): add ThreadItem::DynamicToolCall (#12732)

Previously, clients would call `thread/start` with dynamic_tools set,
and when a model invokes a dynamic tool, it would just make the
server->client `item/tool/call` request and wait for the client's
response to complete the tool call. This works, but it doesn't have an
`item/started` or `item/completed` event.

Now we are doing this:
- [new] emit `item/started` with `DynamicToolCall` populated with the
call arguments
- send an `item/tool/call` server request
- [new] once the client responds, emit `item/completed` with
`DynamicToolCall` populated with the response.

Also, with `persistExtendedHistory: true`, dynamic tool calls are now
reconstructable in `thread/read` and `thread/resume` as
`ThreadItem::DynamicToolCall`.

codex-rs/protocol/src/protocol.rs

@@ -20,6 +20,7 @@ use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::config_types::WindowsSandboxLevel;
 use crate::custom_prompts::CustomPrompt;
+use crate::dynamic_tools::DynamicToolCallOutputContentItem;
 use crate::dynamic_tools::DynamicToolCallRequest;
 use crate::dynamic_tools::DynamicToolResponse;
 use crate::dynamic_tools::DynamicToolSpec;
@@ -1053,6 +1054,8 @@ pub enum EventMsg {
 
     DynamicToolCallRequest(DynamicToolCallRequest),
 
+    DynamicToolCallResponse(DynamicToolCallResponseEvent),
+
     SkillRequestApproval(SkillRequestApprovalEvent),
 
     ElicitationRequest(ElicitationRequestEvent),
@@ -1781,6 +1784,27 @@ pub struct McpToolCallEndEvent {
     pub result: Result<CallToolResult, String>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS, PartialEq)]
+pub struct DynamicToolCallResponseEvent {
+    /// Identifier for the corresponding DynamicToolCallRequest.
+    pub call_id: String,
+    /// Turn ID that this dynamic tool call belongs to.
+    pub turn_id: String,
+    /// Dynamic tool name.
+    pub tool: String,
+    /// Dynamic tool call arguments.
+    pub arguments: serde_json::Value,
+    /// Dynamic tool response content items.
+    pub content_items: Vec<DynamicToolCallOutputContentItem>,
+    /// Whether the tool call succeeded.
+    pub success: bool,
+    /// Optional error text when the tool call failed before producing a response.
+    pub error: Option<String>,
+    /// The duration of the dynamic tool call.
+    #[ts(type = "string")]
+    pub duration: Duration,
+}
+
 impl McpToolCallEndEvent {
     pub fn is_success(&self) -> bool {
         match &self.result {