fix(tui): avoid stuck QUEUED badges from orphan assistant messages

[MaheshBhushan]

Issue for this PR

Closes #16856

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes the stuck QUEUED badge when an old orphan assistant message exists in session history.

Before: pending used indLast over all messages for assistant entries missing ime.completed. If an older crashed/orphaned assistant message was still incomplete, pending pointed to it forever, so later user messages looked queued even when idle.

After: pending only considers the tail message, and only if it is an assistant message that is still incomplete and has no error.

Why this works: only the tail assistant message can represent current in-flight generation; historical orphan records should not drive live queued state.

How did you verify your code works?

• Reproduced logic path by inspecting the current queued condition in UserMessage (message.id > pending).
• Verified the new memo returns undefined when the last message is not an active assistant response.
• Confirmed this prevents stale mid-history assistant records from setting global queued status.

Screenshots / recordings

Not included (small logic-only TUI state fix).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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