fix: handle malformed user/view payloads in extract_team_id

[saschabuehrle]

Summary

This prevents extract_team_id() from raising KeyError when malformed payloads contain user or view objects without a team_id key.

Changes

• use .get("team_id") for payload["user"] and payload["view"] lookup in extract_team_id
• extend invalid-payload coverage in test_extraction_functions_invalid_dict_keys

Why

Slack requests should still be rejected by request verification, but malformed traffic should not crash context extraction before middleware runs.

Testing

• python3 -m compileall -q slack_bolt/request/internals.py tests/slack_bolt/request/test_internals.py

Fixes #1447

Greetings, saschabuehrle