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/protocol/src/protocol.rs

@@ -17,6 +17,8 @@ use crate::config_types::CollaborationMode;
 use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::custom_prompts::CustomPrompt;
+use crate::dynamic_tools::DynamicToolCallRequest;
+use crate::dynamic_tools::DynamicToolResponse;
 use crate::items::TurnItem;
 use crate::message_history::HistoryEntry;
 use crate::models::BaseInstructions;
@@ -216,6 +218,14 @@ pub enum Op {
         response: RequestUserInputResponse,
     },
 
+    /// Resolve a dynamic tool call request.
+    DynamicToolResponse {
+        /// Call id for the in-flight request.
+        id: String,
+        /// Tool output payload.
+        response: DynamicToolResponse,
+    },
+
     /// Append an entry to the persistent cross-session message history.
     ///
     /// Note the entry is not guaranteed to be logged if the user has
@@ -750,6 +760,8 @@ pub enum EventMsg {
 
     RequestUserInput(RequestUserInputEvent),
 
+    DynamicToolCallRequest(DynamicToolCallRequest),
+
     ElicitationRequest(ElicitationRequestEvent),
 
     ApplyPatchApprovalRequest(ApplyPatchApprovalRequestEvent),