Support standalone tool outputs in turn/start (#41002)

## What changed

- Add an optional `toolOutput` to `turn/start` for starting or steering a turn with a named function-call output instead of user input.
- Emit standalone outputs as `functionCallOutput` thread items, persist them in durable history, and include them in resumed threads.
- Validate that tool outputs have a name, are not combined with nonempty `input`, satisfy the text-size limit, and use supported image URLs.

## Testing

- Cover standalone tool-output notifications, model input, and resumed history in both legacy and paginated history modes.
- Cover the text-size limit for structured tool output.

GitOrigin-RevId: a1eeec7b9b7de67b1a1c5e829a54552b85ec118b

codex-rs/protocol/src/items.rs

@@ -5,6 +5,7 @@ use crate::dynamic_tools::DynamicToolCallOutputContentItem;
 use crate::mcp::CallToolResult;
 use crate::memory_citation::MemoryCitation;
 use crate::models::ContentItem;
+use crate::models::FunctionCallOutputBody;
 use crate::models::ImageDetail;
 use crate::models::MessagePhase;
 use crate::models::ResponseItem;
@@ -43,6 +44,7 @@ use ts_rs::TS;
 #[ts(tag = "type")]
 pub enum TurnItem {
     UserMessage(UserMessageItem),
+    FunctionCallOutput(FunctionCallOutputItem),
     HookPrompt(HookPromptItem),
     AgentMessage(AgentMessageItem),
     Plan(PlanItem),
@@ -83,6 +85,16 @@ pub struct UserMessageItem {
     pub content: Vec<UserInput>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
+pub struct FunctionCallOutputItem {
+    pub id: String,
+    pub name: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub namespace: Option<String>,
+    pub output: FunctionCallOutputBody,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq, Eq)]
 pub struct HookPromptItem {
     pub id: String,
@@ -677,6 +689,7 @@ impl TurnItem {
     pub fn id(&self) -> String {
         match self {
             TurnItem::UserMessage(item) => item.id.clone(),
+            TurnItem::FunctionCallOutput(item) => item.id.clone(),
             TurnItem::HookPrompt(item) => item.id.clone(),
             TurnItem::AgentMessage(item) => item.id.clone(),
             TurnItem::Plan(item) => item.id.clone(),

codex-rs/protocol/src/legacy_events.rs

@@ -520,7 +520,7 @@ impl TurnItem {
     pub fn as_legacy_events(&self, show_raw_agent_reasoning: bool) -> Vec<EventMsg> {
         match self {
             TurnItem::UserMessage(item) => vec![item.as_legacy_event()],
-            TurnItem::HookPrompt(_) => Vec::new(),
+            TurnItem::FunctionCallOutput(_) | TurnItem::HookPrompt(_) => Vec::new(),
             TurnItem::AgentMessage(item) => item.as_legacy_events(),
             TurnItem::Plan(_) => Vec::new(),
             TurnItem::CommandExecution(_)