feat: support underscores in block names

[naaa760]

fix: #19591

Overview

Support underscores in block names for consistency with variable naming rules.

Changes

• Updated BlockDocumentName validation to allow underscores
• Modified UI regex and error message
• Updated tests to reflect new validation rules

Checklist

• References related issue
• Includes unit tests

Testing

• ✅ Underscores now allowed: my_block, test-123_abc
• ✅ Invalid chars still rejected: uppercase, spaces, special chars
• ✅ Consistent validation across blocks and variables

[codspeed-hq]

CodSpeed Performance Report

Merging #19592 will not alter performance

_{Comparing naaa760:feat/supp-under (57e1b11) with main (26aa8c6)}

Summary

✅ 2 untouched

[zzstoatzz]

hi @naaa760 - thank you for the PR! however this is likely someone we'll need to deliberate internally, as this has implications across the full stack and prefect cloud