feat(models): add data_visualization Block Kit block

[zimeg]

Summary

Adds the net-new data_visualization Block Kit block to the SDK's block models. The block renders data as a pie, bar, area, or line chart.

• New DataVisualizationBlock (type: "data_visualization") in slack_sdk/models/blocks/blocks.py with title (required, max 50 chars), chart (required), and optional block_id.
• The chart object is accepted as a dict (consistent with how TableBlock/DataTableBlock model their nested structures): a pie chart carries segments; bar/area/line carry series plus axis_config.
• Validation: title presence + length (<=50), chart presence, and chart.type membership in {pie, bar, area, line}.
• Registered in Block.parse() and exported from slack_sdk/models/blocks/__init__.py (DataVisualizationBlock in both the import and __all__).
• Tests in tests/slack_sdk/models/test_blocks.py mirroring the existing DataTableBlock coverage (doc example, bar chart with block_id, line chart with negatives, required-arg and validation cases).

Docs

https://docs.slack.dev/reference/block-kit/blocks/data-visualization-block

Validation steps run

• black --line-length 125 on the touched files: no changes (already compliant).
• pytest tests/slack_sdk/models/test_blocks.py: 97 passed (7 new DataVisualizationBlock cases included).
• mypy slack_sdk/models/blocks/blocks.py: success, no issues.
• Smoke test: construct + to_dict() round-trip and Block.parse() resolve to DataVisualizationBlock.

No changeset added: this repo does not use .changeset/ (version is managed via slack_sdk/version.py).

Co-Authored-By: Claude svc-devxp-claude@slack-corp.com

[codecov]

Codecov Report

❌ Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.17%. Comparing base (e23b181) to head (e1e9aed).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
slack_sdk/models/blocks/blocks.py	98.21%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1899      +/-   ##
==========================================
+ Coverage   84.14%   84.17%   +0.02%     
==========================================
  Files         117      117              
  Lines       13356    13412      +56     
==========================================
+ Hits        11238    11289      +51     
- Misses       2118     2123       +5     

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