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/thread-store/src/local/thread_history.rs

@@ -518,6 +518,7 @@ WHERE thread_id = ?
                 ..
             }
             | ThreadItem::HookPrompt { .. }
+            | ThreadItem::FunctionCallOutput { .. }
             | ThreadItem::Plan { .. }
             | ThreadItem::Reasoning { .. }
             | ThreadItem::CommandExecution { .. }

codex-rs/thread-store/src/local/thread_history/search.rs

@@ -332,6 +332,7 @@ fn searchable_text(item: &ThreadItem) -> Option<Cow<'_, str>> {
             (!text.is_empty()).then_some(Cow::Owned(text))
         }
         ThreadItem::HookPrompt { .. }
+        | ThreadItem::FunctionCallOutput { .. }
         | ThreadItem::Plan { .. }
         | ThreadItem::Reasoning { .. }
         | ThreadItem::CommandExecution { .. }