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/src/dynamic_tools.rs

@@ -0,0 +1,58 @@
+use codex_app_server_protocol::DynamicToolCallResponse;
+use codex_core::CodexThread;
+use codex_protocol::dynamic_tools::DynamicToolResponse as CoreDynamicToolResponse;
+use codex_protocol::protocol::Op;
+use std::sync::Arc;
+use tokio::sync::oneshot;
+use tracing::error;
+
+pub(crate) async fn on_call_response(
+    call_id: String,
+    receiver: oneshot::Receiver<serde_json::Value>,
+    conversation: Arc<CodexThread>,
+) {
+    let response = receiver.await;
+    let value = match response {
+        Ok(value) => value,
+        Err(err) => {
+            error!("request failed: {err:?}");
+            let fallback = CoreDynamicToolResponse {
+                call_id: call_id.clone(),
+                output: "dynamic tool request failed".to_string(),
+                success: false,
+            };
+            if let Err(err) = conversation
+                .submit(Op::DynamicToolResponse {
+                    id: call_id.clone(),
+                    response: fallback,
+                })
+                .await
+            {
+                error!("failed to submit DynamicToolResponse: {err}");
+            }
+            return;
+        }
+    };
+
+    let response = serde_json::from_value::<DynamicToolCallResponse>(value).unwrap_or_else(|err| {
+        error!("failed to deserialize DynamicToolCallResponse: {err}");
+        DynamicToolCallResponse {
+            output: "dynamic tool response was invalid".to_string(),
+            success: false,
+        }
+    });
+    let response = CoreDynamicToolResponse {
+        call_id: call_id.clone(),
+        output: response.output,
+        success: response.success,
+    };
+    if let Err(err) = conversation
+        .submit(Op::DynamicToolResponse {
+            id: call_id,
+            response,
+        })
+        .await
+    {
+        error!("failed to submit DynamicToolResponse: {err}");
+    }
+}