When a chat turn's model stream fails mid-response (e.g. a transport
timeout), the streamed-so-far output is no longer dropped. chat.agent
passes the recovered partial to onTurnComplete, and chat.createSession
accumulates it before turn.complete() rethrows, so it survives for
persistence even when hydrateMessages disables boot-time replay recovery.
The turn is still reported as errored.

Commit the recovered partial to the canonical accumulator on the chat.agent
error path so the next turn and the reboot snapshot both carry it, matching
the success path. Fold queued response parts into the manual-loop error
partial too. Add a continuation regression test.

Replace a same-id continuation partial in place instead of dropping it as a
dup, commit the errored user message unconditionally so it reaches the next
live turn, and push only the appended partial's model messages to preserve a
prior turn's compaction (reconvert only when replacing or folding).

…tep throws

The chat.agent try block spans the whole turn, so a throw from a post-response
step (a customer onBeforeTurnComplete/onTurnComplete hook, a late conversion)
lands in the error handler after the response was already committed. Track a
per-turn responseCommitted flag and keep capturedPartialResponse pointed at the
enriched committed message, so the error path reports it without re-recovering a
raw partial and clobbering the committed message, its queued data parts, or a
prior turn's compaction. Also use truthy id checks to match the success path.

The error-path onTurnComplete added the partial to newUIMessages/uiMessages but
left newMessages (the model-message delta) empty, so apps persisting the model
delta missed the partial. Populate it from the same conversion.

Apply cleanupAbortedParts to the partial recovered on a source-stream failure,
in both the managed and manual loops. A transport error leaves the same
incomplete-part state as a user stop (streaming text not finalized, dangling
tool calls), so the partial is cleaned the same way: text is kept and
unfinished tool parts are dropped, keeping the persisted UI history and the
model view consistent. Also reset the error event's newMessages delta when the
model-message conversion fails, so it can't advertise messages that were rolled
back.