Let image generation extension hosts control output persistence (#29711)

## Why

Some extension hosts need generated images returned without writing them
to the local filesystem or giving the model a local path.

## What changed

**tl;dr**: we now conduct all extension operations in the image gen
extension

- Let hosts provide an optional image save root when installing the
extension.
- Save images and return path hints only when a save root is configured.
- Return image data without saving or adding a path hint when no save
root is configured.
- Preserve the extension-provided `saved_path` instead of persisting
extension images again in core.
- Leave built-in image generation unchanged.

## Validation

- `just test -p codex-image-generation-extension`
- `just test -p codex-app-server
standalone_image_generation_returns_saved_path_hint_to_model`
- `just test -p codex-core
extension_tool_uses_granted_turn_permissions_without_local_persistence`
- `just test -p codex-core tools::handlers::extension_tools::tests`
- tested on CODEX CLI on both save_root: CODEX_HOME and None 
- tested on CODEX APP on both as well

codex-rs/tools/src/tool_call.rs

@@ -43,12 +43,12 @@ pub enum ExtensionTurnItem {
 /// Host-provided capability for extension tools to emit visible turn items.
 ///
 /// Implementations route lifecycle events through the host's normal item event
-/// pipeline, including any persistence and client delivery owned by the host.
+/// pipeline and client delivery.
 pub trait TurnItemEmitter: Send + Sync {
     /// Emits the beginning of one visible turn item.
     fn emit_started<'a>(&'a self, item: ExtensionTurnItem) -> TurnItemEmissionFuture<'a>;
 
-    /// Emits one visible turn item after host-owned finalization.
+    /// Emits one completed visible turn item.
     fn emit_completed<'a>(&'a self, item: ExtensionTurnItem) -> TurnItemEmissionFuture<'a>;
 }