Route standalone image generation through host finalization md (#25176)

## Why

Standalone image-generation extensions emitted turn items through the
low-level event path, bypassing host-owned finalization such as image
persistence and contributor processing. At the same time, the
generated-image save-path hint must remain visible to the model through
the extension tool's `FunctionCallOutput`, rather than the legacy
built-in developer-message path.

## What changed

- Extended `ExtensionTurnItem` to support image-generation items while
keeping the extension-facing emitter API limited to `emit_started` and
`emit_completed`.
- Routed extension completion through core `finalize_turn_item`, so
standalone image-generation items receive host-owned processing and
persisted `saved_path` values before publication.
- Kept legacy built-in image generation on its existing
developer-message hint path, while standalone image generation returns
its deterministic saved-path hint in `FunctionCallOutput`.
- Shared the image artifact path and output-hint formatting used by core
and the image-generation extension.
- Passed thread identity through extension tool calls so standalone
image generation can construct the same intended artifact path as core.
- Added an app-server integration test covering real standalone image
generation, saved artifact publication, model-visible output hint
wiring, and absence of the legacy developer-message hint.

## Validation

- `just fmt`
- `just test -p codex-image-generation-extension`
- `just test -p codex-web-search-extension`
- `just test -p codex-goal-extension`
- `just test -p codex-memories-extension`
- Targeted `codex-core` tests for image save history, extension
completion finalization, and contributor execution
- `just test -p codex-app-server
standalone_image_generation_returns_saved_path_hint_to_model`
- `just fix -p codex-core`
- `just fix -p codex-image-generation-extension`
- `just bazel-lock-update`
- `just bazel-lock-check`

codex-rs/tools/src/lib.rs

@@ -64,7 +64,6 @@ pub use responses_api::mcp_tool_to_responses_api_tool;
 pub use responses_api::tool_definition_to_responses_api_tool;
 pub use tool_call::ConversationHistory;
 pub use tool_call::ExtensionTurnItem;
-pub use tool_call::ImageGenerationCompletionFuture;
 pub use tool_call::NoopTurnItemEmitter;
 pub use tool_call::ToolCall;
 pub use tool_call::TurnItemEmissionFuture;