fix(assistant): get_thread_context calls store.find() for user_message events

[srtaalej]

Summary

Fixes get_thread_context() returning None in user_message handlers when assistant_thread is present in the payload but only contains an action_token (no context).
Closes #1452

Testing

from slack_bolt import App
from slack_bolt import Assistant, Say

assistant = Assistant()  # default store or custom — same result

@assistant.thread_started
def handle_thread_started(say, save_thread_context, payload):
    thread_context = payload["assistant_thread"]["context"]
    save_thread_context(thread_context) 
    say("Hi!")

@assistant.thread_context_changed
def handle_context_changed(save_thread_context, payload):
    new_context = payload["assistant_thread"]["context"]
    save_thread_context(new_context) 

@assistant.user_message
def handle_user_message(say, get_thread_context):
    ctx = get_thread_context()  # should work now
    print(ctx)

app = App(token="xoxb-...", signing_secret="...")
app.use(assistant)

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• 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 ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (98a8f59) to head (99e38ac).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1453   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files         229      229           
  Lines        7266     7266           
=======================================
  Hits         6635     6635           
  Misses        631      631           

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[WilliamBergamin]

Look good 💯

Left one comment around testing, what do you think about leaving the current tests unchanged and instead add new tests for this case?

[WilliamBergamin]

What do you think about leaving the old test cases untouched and adding new ones for this logic?

[WilliamBergamin]

Nice fix 🚀 🥇

Left 2 non blocking comment let me know what you think

[rayterrill]

@srtaalej any idea when we can expect a fix for this? 🙏