fix(desktop): preserve redirect reply order

[0809android]

Summary

Fix reply ordering when a prompt is redirected during an active response. The existing partial reply is sealed, the corrected user prompt stays after it, and the retried reply starts after the correction.

Validation

• 119 targeted UI tests passed
• Verified in Hermes Local Fix on macOS

[spfcraze]

This was generated by AI during triage.

Summary:
The new index > latestUserIndex guard excludes the sealed partial from the completion fallback, so the aborted turn's late message.complete — the race the diff's own comment acknowledges — appends a duplicate assistant row after the correction instead of settling the partial in place.

Problems:

• The redirect path seals the active stream (pending: false, interim: true), appends the correction at the tail, and sets streamId: null before awaiting the gateway. A message.complete for the aborted turn therefore reaches completeAssistantMessage — which reads const streamId = state.streamId — with the fallback path, not the streamId match.
• The fallback now only matches assistant rows with index > latestUserIndex, and the sealed partial sits before the correction's user row, outside that window. The pre-existing interimBoundaryPending && finalContinuesInterim branch in apps/desktop/src/app/session/hooks/use-message-stream/index.ts — built to settle a sealed interim the completion continues — never sees the partial.
• With no fallback match, a non-empty finalText appends newAssistantFromCompletion() after the correction: a second assistant row carrying the aborted turn's text.

Solution:
Keep the sealed partial eligible for the finalContinuesInterim settle when the redirect set interimBoundaryPending; the prefix match already keeps the retried turn's own completion from settling the partial, so the acknowledged late completion completes it in place instead of duplicating it.

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Checked against 2b52de8 — the tip of agent/desktop-message-order when this was written — and 35cbad5, main at the same moment.

[0809android]

Closing after re-checking the Desktop history and current intent. #69739 deliberately established the correction-before-redirected-reply contract, and #73882 was closed for reversing that ordering. This PR changes the same contract without an issue or maintainer alignment, and the current patch also leaves a late-completion duplication race. I should not ask maintainers to review a transcript-contract redesign that was not agreed first.