get_delta_channel_history

Walk the parent chain returning per-channel writes + seed.

For each requested channel, walks ancestors of the checkpoint identified by config (following parent_config) and accumulates pending_writes for that channel. The walk terminates per-channel at the nearest ancestor whose channel_values[ch] is populated; that value is returned as seed. If the walk reaches the root without finding a stored value, seed is omitted from that channel's entry — the consumer treats the absence as "start empty."

Walks the parent chain (not list(before=...)): for forked threads, only on-path ancestors contribute.

The default implementation walks get_tuple + parent_config once for all channels — each ancestor visited once, not once per channel. Savers with direct storage access (InMemorySaver, PostgresSaver) override for performance; the return contract is fixed here.