fix(notifications): make routine errors non-intrusive

[fcoury-oai]

Unknown Ex commands are immediate feedback about input the user just submitted. Treating them as acknowledgment-required errors leaves a typo active indefinitely, lets it resurface after newer messages, and makes the bottom-line attention badge imply an unresolved problem.

This change keeps unknown commands styled as errors and retained in message history, while classifying them as quiet, transient feedback. Attention overrides now select the matching foreground lifetime, so quiet and missed-only notices expire while acknowledgment-required conditions remain active.

How to Test

1. Enter an unknown command such as :not-a-command. Confirm the red error appears without a needs attention or :messages badge, disappears after the transient display period, and remains available in message history.
2. In an unnamed modified buffer, run :write. Confirm No file name still carries the needs attention badge, preserving the non-happy-path behavior for an unresolved save failure.
3. Run the focused notification coverage: cargo test --lib notification::tests, cargo test --lib editor::notifications::tests, and cargo test --test notifications.