fix(outbound): persist mirror route only after successful delivery (#…

fix(outbound): persist mirror route only after successful delivery (#124459)

A failed message send (e.g. missing channel credentials on a fresh
install) durably rewrote the agent's folded main session route before
delivery was even attempted: prepareOutboundMirrorRoute called
ensureOutboundSessionEntry pre-send, stamping delivery.route/origin and
minting a conversations-registry row for the never-reached target. The
Control UI then showed the phantom channel identity and the composer
bound to a dead conversation.

Route resolution stays read-only in prepareOutboundMirrorRoute; the
durable write commits once per send at the first success proof:
identified platform evidence (onDeliveryResult) or plugin action
acceptance (onPluginSendAccepted), both before the in-delivery
transcript mirror so first-contact routes still create their session
row, with a post-return safety net for adapters whose results carry no
platform identity. The gateway send RPC sibling had the same pre-send
persistence and gets the same commit-on-evidence ordering.

Regression test proves a failed send leaves the seeded main-session
origin and conversation identity untouched while a successful send
still persists the mirror route; fails on pre-fix code.