feat: add plan_update chunk for chat streaming

[zimeg]

Summary

This PR adds the plan_update chunk for updating the plan block title in chat streams.

Testing

The following shows the plan block title updating after each few seconds:

import time
# ...

streamer = client.chat_stream(
    channel="C0123456789",
    recipient_team_id="T0123456789",
    recipient_user_id="U0123456789",
    thread_ts="17000000000.00000",
    task_display_mode="plan",
)

streamer.append(
    chunks=[
        MarkdownTextChunk(
            text="Hello.\nI have received the task. ",
        ),
        MarkdownTextChunk(
            text="This task appears manageable.\nThat is good.",
        ),
        TaskUpdateChunk(
            id="001",
            title="Understanding the task...",
            status="in_progress",
            details="- Identifying the goal\n- Identifying constraints",
        ),
        TaskUpdateChunk(
            id="002",
            title="Performing acrobatics...",
            status="pending",
        ),
    ],
)
time.sleep(4)

streamer.append(
    chunks=[
        PlanUpdateChunk(
            title="Adding the final pieces...",
        ),
        TaskUpdateChunk(
            id="001",
            title="Understanding the task...",
            status="complete",
            details="\n- Pretending this was obvious",
            output="We'll continue to ramble now",
        ),
        TaskUpdateChunk(
            id="002",
            title="Performing acrobatics...",
            status="in_progress",
        ),
    ],
)
time.sleep(4)

streamer.stop(
    chunks=[
        TaskUpdateChunk(
            id="002",
            title="Performing acrobatics...",
            status="complete",
            details="- Jumped atop ropes\n- Juggled bowling pins\n- Rode a single wheel too",
        ),
        MarkdownTextChunk(
            text="The crowd appears to be astouned and applauds :popcorn:"
        ),
        PlanUpdateChunk(
            title="Celebrating a job well done!",
        ),
    ],
)

Category

• slack_sdk.models (UI component builders)
• /docs (Documents)
• tests/integration_tests (Automated tests for this library)

Requirements

• 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.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.93%. Comparing base (7c32814) to head (4d355da).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
slack_sdk/models/messages/chunk.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           zimeg-feat-ai-apps-chunks-streamer    #1821      +/-   ##
======================================================================
- Coverage                               83.93%   83.93%   -0.01%     
======================================================================
  Files                                     116      116              
  Lines                                   13192    13203      +11     
======================================================================
+ Hits                                    11073    11082       +9     
- Misses                                   2119     2121       +2     

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[srtaalej]

🌟

[srtaalej]

thanks for getting clarification on what this chunk should look like 😸