Route standalone image generation through host finalization md

[won-openai]

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