Fix system notification command rendering as literal backtick (#318601) (#319052)

* Fix system notification command rendering as literal backtick (#318601)

Terminal background-execution system notification labels wrapped the raw
command in a single-backtick inline code span. Multi-line commands contain
blank lines, which break an inline code span and cause the leading backtick
to render literally instead of as code.

Reuse the existing safe pattern (buildCommandDisplayText to collapse newlines
and truncate, appendEscapedMarkdownInlineCode to fence safely) so the command
always renders as inline code.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Simplify command display to first line + ellipsis (#318601)

Replace newline-to-space collapsing in buildCommandDisplayText with
first-line-only behavior: keep only the first line and append an
ellipsis when the command spans multiple lines (or when the first line
itself exceeds 80 characters). This is cleaner for UI labels than
joining multi-line commands into a single long line of spaces.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Scope first-line truncation to system notification site

Revert the change to buildCommandDisplayText (it's used by other
callers that expect the prior collapse-newlines behavior). Do the
first-line + ellipsis truncation locally in _registerCompletionNotification
where the system notification label is built.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Extract buildCompletionNotificationCommand helper + tests (#318601)

Move the first-line + ellipsis logic into a small exported helper next
to the call site so it can be tested directly. Restore the trim/escape-
artifact cleanup and 80-character truncation by running the first line
through buildCommandDisplayText (which is a no-op for newlines once the
input is already a single line).

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Use horizontal ellipsis character in completion notification

Replace the three-dot "..." with the proper horizontal ellipsis
character (U+2026) in buildCompletionNotificationCommand. Updated the
truncation path to slice to 79 chars + 1-char ellipsis so the total
length remains 80.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>