Add persist_docs support for semantic views

[RyutoYoda]

Summary

Implements relation-level persist_docs functionality for semantic views, addressing the "persist_docs not supported" limitation mentioned in the README and resolving the TODO comment in the materialization.

• ✅ Automatically adds model descriptions from schema.yml as COMMENT clauses to Semantic View DDL
• ✅ Detects and avoids conflicts with existing inline COMMENT syntax
• ✅ Follows the existing copy_grants pattern for consistency
• ✅ Includes comprehensive test coverage for enabled and disabled scenarios

Usage Example

# schema.yml
models:
  - name: my_semantic_view
    description: "Customer metrics for analysis"

# model config
{{ config(
    materialized='semantic_view',
    persist_docs={'relation': true}
) }}

Results in DDL with: COMMENT='Customer metrics for analysis'

Test Plan

• Added test model with persist_docs enabled
• Added positive test verifying comment is added when enabled
• Added negative test verifying comment is NOT added when disabled
• Verified existing inline COMMENT syntax is preserved
• Tested on actual Snowflake environment

Changes

• Updated macros/relations/semantic_view/create.sql with new append_comment_if_missing macro
• Modified macros/materializations/semantic_view.sql to remove TODO comment
• Enhanced README with persist_docs documentation and usage examples
• Added integration test model and test cases

[Daniel-Wiszowaty]

Hey @sfc-gh-yutliu could we push this? 🙏🏻

[ahossain-lithic]

This would be great to have!

[RyutoYoda]

Hey @sfc-gh-yutliu 👋 Just checking in — any chance this could get a review when you have a moment? Happy to make any changes if needed. Thanks!

[LizzyHunter]

This is not only nice to have but a feature that can make the difference between keeping on using dbt / metricflow semantic layer VS fully adopting Snowflake solutions and downstream AI capabilities. Any chance this can be pushed? 🙏

[sfc-gh-sarur]

@RyutoYoda please resolve the conflicts in this PR before merging, thanks

[jheath5]

@sfc-gh-sarur @RyutoYoda anything to do to get this merged? Not familiar on how to contribute/finish for this, but as @LizzyHunter said this will increase adoption of Snowflake semantic views for teams like mine that use dbt