Fix get_task_output empty results for background tasks while preserving single-run reused terminal behavior

[meganrogge]

Fixes #304057.

get_task_output could return empty output even when the task terminal had content because output collection could start from a fresh marker created at read time, which for completed/background tasks can point to the end of the buffer.

Now, in getTaskOutputTool, pass a start-marker map only for background tasks. For those background/watch tasks, set each terminal entry to undefined so collection reads the current terminal buffer instead of from a newly created marker.

Keep existing behavior for single-run tasks, so reused-terminal protection remains and old buffer content is not pulled in.

[Copilot]

Pull request overview

Fixes get_task_output returning empty output for background/watch tasks by controlling the start marker behavior used when collecting terminal buffer contents.

Changes:

• For background tasks, passes a startMarkersByTerminalInstanceId map to collectTerminalResults with per-terminal entries set to undefined so output reads from the current terminal buffer (instead of from a newly created end-of-buffer marker).
• Preserves existing marker behavior for non-background (single-run) tasks to avoid pulling in unrelated old buffer content when terminals are reused.

[meganrogge]

@copilot open a new pull request to apply changes based on the comments in this thread

[Copilot]

@meganrogge I've opened a new pull request, #304124, to work on those changes. Once the pull request is ready, I'll request review from you.