`extract_team_id` crashes with `TypeError when payload["user"]` is a string (e.g. `member_joined_channel` events)

Reproducible in:

The slack_bolt version

slack-bolt==1.27.0
slack-sdk==3.39.0

Python runtime version

Python 3.14.0

OS info

ProductName:    macOS
ProductVersion: 26.3
BuildVersion:   25D125
Darwin Kernel Version 25.3.0: Wed Jan 28 20:53:05 PST 2026; root:xnu-12377.81.4~5/RELEASE_ARM64_T6020

Steps to reproduce:

1. Register a member_joined_channel listener in a Slack Bolt app:

@app.event("member_joined_channel")
def handle_channel_join(event, client, say):
    say("Hello!")

2. Add the bot to any Slack channel.

3. Observe the incoming event payload — Slack sends "user" as a plain string (the user ID):

{
  "type": "member_joined_channel",
  "user": "U0123456789",
  "channel": "C0123456789",
  "team": "T0123456789"
}

Expected result:

The listener is invoked normally.

Actual result:

The request crashes before the listener is reached, with:

TypeError: string indices must be integers, not 'str'

File "slack_bolt/adapter/aws_lambda/handler.py", line 87, in to_bolt_request
    return BoltRequest(
File "slack_bolt/request/request.py", line 69, in __init__
    self.context = build_context(BoltContext(context if context else {}), self.body)
File "slack_bolt/request/internals.py", line 276, in build_context
    team_id = extract_team_id(body)
File "slack_bolt/request/internals.py", line 116, in extract_team_id
    return payload["user"]["team_id"]

Root cause

extract_team_id in slack_bolt/request/internals.py assumes payload["user"] is always a dict:

if payload.get("user") is not None:
    return payload["user"]["team_id"]

But member_joined_channel sends "user" as a plain string (the user ID). Accessing "U0123456789"["team_id"] raises the TypeError.

The same function already handles payload["team"] correctly with an isinstance check:

if payload.get("team") is not None:
    team = payload.get("team")
    if isinstance(team, str):
        return team
    elif team and "id" in team:
        return team.get("id")

The same guard is missing for payload["user"]. This pattern (subscribing to member_joined_channel and checking if the joined user is the bot) is what maintainers recommend in #710, so users following that guidance will hit this bug.

Comienza en slack_bolt/request/internals.py, en extract_team_id, y después inspecciona el manejo existente del tipo de payload["team"]. Reproduce el payload de member_joined_channel con un valor de tipo string para user y verifica que el procesamiento de la request llegue al listener sin un TypeError. Se considera terminado cuando los campos user con valores de tipo string ya no provocan un fallo en la extracción del ID del equipo y los formatos de payload existentes siguen funcionando.