Fix #1297 Building user_auth_blocks with slack_sdk.models class objects for chat.unfurl API call fails

[seratch]

Summary

This pull request resolves #1297

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

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

[codecov]

Codecov Report

Merging #1301 (d417dc9) into main (3610592) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1301      +/-   ##
==========================================
- Coverage   85.79%   85.78%   -0.02%     
==========================================
  Files          84       84              
  Lines        7845     7846       +1     
==========================================
  Hits         6731     6731              
- Misses       1114     1115       +1     

Impacted Files	Coverage Δ
slack_sdk/web/legacy_client.py	83.26% <100.00%> (+0.01%)	⬆️
slack_sdk/socket_mode/builtin/internals.py	72.10% <0.00%> (-0.43%)	⬇️

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[seratch]

Thanks for the review!