feat: Support individual subagent cancellation

[tomjw64]

What does this PR do?

Closes #13916

Adds support for interrupting subagent tasks with a clickable button or the session_interrupt keybinding in the TUI. The motivation behind this work is twofold:

1. Face-value: Support this feature so that individual subagents can be cancelled when a group of subagents is run without cancelling everything
2. Support this feature as a prerequisite to background subagents. This can give us a way to abort background subagents without creeping the scope of that feature addition too much. See: [feat] True Async/Background Sub-Agent Delegation #5887, feat(opencode): Support background subagents #13261

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How did you verify your code works?

Ran the following prompt repeatedly:

Run 2 general subagents in parallel. Each one should use bash to
sleep for an amount of time and then print out a random number. Have them sleep
for 30 seconds each. Then print out a chart to be
updated as each task notifies you of its completion.

And tested interrupt behavior including:

• That interrupt must be re-confirmed after changing session views
• That old interrupt confirmation timeouts don't overlap with the current timeout
• That subagents can be interrupted individually, without affecting the others
• That the primary agent continues as expected when all subagents terminate, via interrupts or successful completion
• That the primary agent can be interrupted
• That the parent agent sees the message that the task was cancelled by the user