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/app-server/src/extensions.rs

@@ -76,7 +76,9 @@ where
     codex_mcp_extension::install(&mut builder);
     codex_mcp_extension::install_executor_plugins(&mut builder, environment_manager);
     codex_web_search_extension::install(&mut builder, auth_manager.clone());
-    codex_image_generation_extension::install(&mut builder, auth_manager);
+    codex_image_generation_extension::install(&mut builder, auth_manager, |config: &Config| {
+        Some(config.codex_home.clone())
+    });
     let skill_providers = codex_skills_extension::SkillProviders::new()
         .with_executor_provider(executor_skill_provider)
         .with_orchestrator_provider(Arc::new(