[FEATURE]: add task-parallel tool to fan out independent subtasks concurrently

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Describe the enhancement you want to request

Long-running agent tasks are slow in wall-clock time because independent subtasks run sequentially. The task tool already supports background=true for async subagents, and the BackgroundJob registry forks each job into its own scope, so parallel execution is already possible — but the model has to orchestrate it manually by issuing multiple task calls with background=true in one message, and then collect each background notification. That's error-prone: the model often forgets to wait, or polls, or duplicates work.

What I'd like to see: a task-parallel tool that takes a list of 2-5 independent subtasks (each with its own description, prompt, and subagent type), launches them all concurrently, waits for every one to finish, and returns a single combined summary. Each subtask runs in its own subagent session (reusing the existing deriveSubagentSessionPermission + sessions.create + ops.prompt machinery from the task tool), so the wall-clock time is roughly the longest subtask instead of the sum.

Benefits:

• Turns a "run N independent things" task from N sequential subagent runs into one parallel fan-out with a single result.
• Reuses the existing subagent/permission/background infrastructure instead of inventing new execution logic.
• Fails gracefully per-subtask: a failing subtask is reported as error in the summary, the others still complete.

I have a working implementation with unit tests in PR #46990 (opened separately).