Fix #1261 blocks/attachments as str for chat.* API calls should be clearly supported

[seratch]

Summary

This pull request resolves #1261

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.

[seratch]

These tests added as part of #1259 actually are not properly working. I've rewritten the tests to work as we expect.

[codecov]

Codecov Report

Merging #1262 (1262422) into main (ffffc18) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1262   +/-   ##
=======================================
  Coverage   86.58%   86.59%           
=======================================
  Files         111      111           
  Lines       11026    11026           
=======================================
+ Hits         9547     9548    +1     
+ Misses       1479     1478    -1     

Impacted Files	Coverage Δ
slack_sdk/web/async_client.py	85.14% <ø> (ø)
slack_sdk/web/client.py	86.52% <ø> (ø)
slack_sdk/web/legacy_client.py	86.24% <ø> (ø)
slack_sdk/web/internal_utils.py	94.44% <100.00%> (ø)
slack_sdk/socket_mode/builtin/internals.py	72.24% <0.00%> (-0.45%)	⬇️
slack_sdk/socket_mode/websocket_client/__init__.py	85.61% <0.00%> (+1.43%)	⬆️

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