* fix: Branch in new chat loses the question and the branched session (#issue)
- session.branch on the backend now accepts a count param to truncate
the parent history to the clicked message, instead of always forking
the entire transcript. Also returns stored_session_id/messages/info
so the frontend has parity with session.create's response shape.
- branchCurrentSession (open live chat) now slices history from 0
instead of from the clicked message index, so the question preceding
an assistant reply is no longer dropped when branching.
- forkBranch now calls session.branch (not session.create) when
branching an open live chat, since session.create only persists a DB
row lazily on first prompt - a branched chat that nobody types into
never got saved, and vanished as 'session not found' on the next
app restart. branchStoredSession (branching from the sidebar, no
live runtime) keeps using session.create as before.
* test: cover session.branch count truncation and open-chat branching
- backend: assert session.branch with a count param only persists the
first N messages of the live history to the new session.
- frontend: BranchHarness now exposes branchCurrentSession; assert
branching an open chat from a middle message calls session.branch
with the parent session id and the correct trimmed count, instead of
session.create.
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