feat: add task_card and plan_blocks

[srtaalej]

Summary

This PR adds the task_card block and the plan block for usage in standalone messages.

⚠️ This is experimental and requires a toggle to preview!

const parentMessage = await client.chat.postMessage({
      channel: CHANNEL_ID,
      text: '🧵 Starting a streaming conversation with chunks...',
      blocks: [
        {
          type: 'plan',
          plan_id: 'plan-123',
          title: 'My Task',
          tasks: [
            {
              type: 'task_card',
              task_id: 'task-124',
              title: 'Task1',
              status: 'complete'
            },
            {
              type: 'task_card',
              task_id: 'task-125',
              title: 'task2',
              status: 'complete'
            },
          ]
        }
      ]
    });

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.09%. Comparing base (b44cee1) to head (18a68a7).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                     Coverage Diff                      @@
##           feat-ai-apps-thinking-steps    #2471   +/-   ##
============================================================
  Coverage                        93.09%   93.09%           
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  Files                               40       40           
  Lines                            11240    11240           
  Branches                           713      713           
============================================================
  Hits                             10464    10464           
  Misses                             764      764           
  Partials                            12       12           

Flag	Coverage Δ
cli-hooks	95.23% <ø> (ø)
cli-test	94.79% <ø> (ø)
webhook	96.66% <ø> (ø)

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