ViewImageMiddleware persists full base64 image payloads in hide_from_ui
human messages inside checkpoints. All REST endpoints that returned
serialize_channel_values(channel_values) sent these multi-megabyte
payloads to the frontend, freezing the UI on threads with images.
Add strip_data_url_image_blocks() to remove data:-scheme image_url
content blocks from hide_from_ui messages, and
serialize_channel_values_for_api() as a convenience wrapper used by all
six affected call sites across threads, runs, and thread_runs routers.
SSE streaming is unaffected (still uses serialize_channel_values).
Fixes#3496
094296440f