Fix immediate sends with pending link previews

[bytware-alpha]

Why

Sending a link should behave like sending any other message. The bubble should appear immediately; the preview can finish a beat later and still belong to that same bubble.

#4550 already ruled out the cheap fixes: blocking Send is bad UX, and sending the preview later is not what the user chose to send. So this does the actual thing: persist the outgoing row now, keep the preview attached to that send, and only hand the finalized payload to the sender.

This is better than the earlier durable-job shape because it does not teach the job queue about half-built sends. No runnable job exists until the message is final. Ordering is held with a small in-memory barrier, which is the sort of short-lived queueing pattern maintainers have accepted before in #4004 and #2529. Less machinery, cleaner contract.

How

• Keep an in-flight link preview alive when Send wins the race.
• Insert the outgoing message row immediately so the conversation stays stable.
• Hold sender ordering with an in-memory deferred job barrier.
• Attach the preview to the same row, then create the real sender job.
• Keep edit/share approval paths using the same preview handoff.

Test Plan

• xcodebuild test -workspace Signal.xcworkspace -scheme Signal -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.5' -only-testing:SignalServiceKitTests/MessageSenderJobQueueTest/test_deferredJobBlocksLaterJobsUntilStarted -only-testing:SignalTests/ThreadUtilSignalUITest/testPendingLinkPreviewDoesNotBlockOutgoingRowPersistence
• xcodebuild test -workspace Signal.xcworkspace -scheme Signal -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.5' -only-testing:SignalUITests/LinkPreviewFetchStateTest/testLinkPreviewDraftForSendingTaskSurvivesClearingCurrentText
• PATH="/tmp/signal-swiftformat/SwiftFormat/.build/release:$PATH" Scripts/precommit.py --ref origin/main
• git diff --check