* fix(agents): keep merged delivery routes account-bound
mergeDeliveryContext gated route-field crossing on channel only, so a
completion origin that knew its account but not a concrete target
inherited a different account's to/threadId on the same channel. A
subagent, cron, or media completion for bot-a could be addressed to
bot-b's chat but sent through bot-a (cross-account misroute) or dropped.
This restores the account-bound guard added in 1ed8592467 and removed as
collateral by 025db6cf9e (PR #89949); same-account and missing-account
merges still backfill so the media route-pin path is preserved. Restores
the deleted regression test.
* fix(agents): centralize account-bound completion routes
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Co-authored-by: Peter Steinberger <steipete@golden-gate.local>
150ca2fedd