Avoid rendering generated images twice (#34378)

## What changed

- Tell the model that generated images are already displayed to the user and should not be repeated as Markdown images or file links.
- Keep generated-image artifact paths and output hints owned by the image generation extension.
- Centralize the availability check for the image generation tool across feature, provider, namespace-tool, model modality, and authentication requirements.

## Testing

- Verify artifact paths sanitize session and call identifiers.
- Verify the model-facing output hint reports that the generated image is already displayed.

GitOrigin-RevId: 6c865b6a96d6a71bb2c33246d1518a00e7c8b840

codex-rs/core/src/context_manager/history_tests.rs

@@ -1047,7 +1047,6 @@ fn drop_last_n_user_turns_trims_context_updates_above_rolled_back_turn() {
         assistant_msg("session prefix item"),
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
-        developer_msg("Generated images are saved to /tmp as /tmp/image-1.png by default."),
         developer_msg(&format!(
             "{APPS_INSTRUCTIONS_OPEN_TAG}\nROLLED_BACK_APPS_INSTRUCTIONS"
         )),
@@ -1078,7 +1077,6 @@ fn drop_last_n_user_turns_trims_context_updates_above_rolled_back_turn() {
             assistant_msg("session prefix item"),
             user_input_text_msg("turn 1 user"),
             assistant_msg("turn 1 assistant"),
-            developer_msg("Generated images are saved to /tmp as /tmp/image-1.png by default."),
         ]
     );
     assert_eq!(