[tool search] support namespaced deferred dynamic tools (#18413)

Deferred dynamic tools need to round-trip a namespace so a tool returned
by `tool_search` can be called through the same registry key that core
uses for dispatch.

This change adds namespace support for dynamic tool specs/calls,
persists it through app-server thread state, and routes dynamic tool
calls by full `ToolName` while still sending the app the leaf tool name.
Deferred dynamic tools must provide a namespace; non-deferred dynamic
tools may remain top-level.

It also introduces `LoadableToolSpec` as the shared
function-or-namespace Responses shape used by both `tool_search` output
and dynamic tool registration, so dynamic tools use the same wrapping
logic in both paths.

Validation:
- `cargo test -p codex-tools`
- `cargo test -p codex-core tool_search`

---------

Co-authored-by: Sayan Sisodiya <sayan@openai.com>

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

@@ -470,6 +470,7 @@ impl ThreadHistoryBuilder {
     ) {
         let item = ThreadItem::DynamicToolCall {
             id: payload.call_id.clone(),
+            namespace: payload.namespace.clone(),
             tool: payload.tool.clone(),
             arguments: payload.arguments.clone(),
             status: DynamicToolCallStatus::InProgress,
@@ -493,6 +494,7 @@ impl ThreadHistoryBuilder {
         let duration_ms = i64::try_from(payload.duration.as_millis()).ok();
         let item = ThreadItem::DynamicToolCall {
             id: payload.call_id.clone(),
+            namespace: payload.namespace.clone(),
             tool: payload.tool.clone(),
             arguments: payload.arguments.clone(),
             status,
@@ -1975,6 +1977,7 @@ mod tests {
                 codex_protocol::dynamic_tools::DynamicToolCallRequest {
                     call_id: "dyn-1".into(),
                     turn_id: "turn-1".into(),
+                    namespace: Some("codex_app".into()),
                     tool: "lookup_ticket".into(),
                     arguments: serde_json::json!({"id":"ABC-123"}),
                 },
@@ -1982,6 +1985,7 @@ mod tests {
             EventMsg::DynamicToolCallResponse(DynamicToolCallResponseEvent {
                 call_id: "dyn-1".into(),
                 turn_id: "turn-1".into(),
+                namespace: Some("codex_app".into()),
                 tool: "lookup_ticket".into(),
                 arguments: serde_json::json!({"id":"ABC-123"}),
                 content_items: vec![CoreDynamicToolCallOutputContentItem::InputText {
@@ -2004,6 +2008,7 @@ mod tests {
             turns[0].items[1],
             ThreadItem::DynamicToolCall {
                 id: "dyn-1".into(),
+                namespace: Some("codex_app".into()),
                 tool: "lookup_ticket".into(),
                 arguments: serde_json::json!({"id":"ABC-123"}),
                 status: DynamicToolCallStatus::Completed,

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

@@ -613,6 +613,8 @@ pub struct ToolsV2 {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct DynamicToolSpec {
+    #[ts(optional)]
+    pub namespace: Option<String>,
     pub name: String,
     pub description: String,
     pub input_schema: JsonValue,
@@ -623,6 +625,7 @@ pub struct DynamicToolSpec {
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
 struct DynamicToolSpecDe {
+    namespace: Option<String>,
     name: String,
     description: String,
     input_schema: JsonValue,
@@ -636,6 +639,7 @@ impl<'de> Deserialize<'de> for DynamicToolSpec {
         D: serde::Deserializer<'de>,
     {
         let DynamicToolSpecDe {
+            namespace,
             name,
             description,
             input_schema,
@@ -644,6 +648,7 @@ impl<'de> Deserialize<'de> for DynamicToolSpec {
         } = DynamicToolSpecDe::deserialize(deserializer)?;
 
         Ok(Self {
+            namespace,
             name,
             description,
             input_schema,
@@ -4887,6 +4892,7 @@ pub enum ThreadItem {
     #[ts(rename_all = "camelCase")]
     DynamicToolCall {
         id: String,
+        namespace: Option<String>,
         tool: String,
         arguments: JsonValue,
         status: DynamicToolCallStatus,
@@ -6650,6 +6656,7 @@ pub struct DynamicToolCallParams {
     pub thread_id: String,
     pub turn_id: String,
     pub call_id: String,
+    pub namespace: Option<String>,
     pub tool: String,
     pub arguments: JsonValue,
 }
@@ -9303,6 +9310,7 @@ mod tests {
         assert_eq!(
             actual,
             DynamicToolSpec {
+                namespace: None,
                 name: "lookup_ticket".to_string(),
                 description: "Fetch a ticket".to_string(),
                 input_schema: json!({