Add data type descriptions to schema sidebar details

[kmcginnes]

Description

Refactor the schema sidebar detail components to add descriptive headers, an empty state for properties, and shared layout primitives. Also adds a steering rule against testing styling/layout and includes behavioral tests for PropertiesDetails.

• Extract shared Details, DetailsHeader, DetailsDescription layout components to reduce duplication across NodeLabelDetails and EdgeConnectionDetails
• Rename AttributeList to PropertiesDetails and move the section title, count chip, description, and empty state handling into it so both panels get consistent behavior
• Change DetailsTitle from div to h2 for better semantics
• Change the properties count chip variant from primary-subtle to neutral-subtle
• Add JSDoc comments to all exported components in Details.tsx
• Add tests for PropertiesDetails covering empty state, populated list, attribute count, and description text
• Add "Do Not Test Styling or Layout" rule to .kiro/steering/testing.md

Validation

• All existing tests pass (pnpm test)
• New tests pass for PropertiesDetails (4 tests)
• No type errors, lint errors, or formatting issues (pnpm checks)
• Visual review of the schema sidebar in the browser

Related Issues

• Resolves Add Helper Text for Data Property Types in Schema Explorer Sidebar #1519

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0
  license.
• I have run pnpm checks to ensure code compiles and meets standards.
• I have run pnpm test to check if all tests are passing.
• I have covered new added functionality with unit tests if necessary.
• I have added an entry in the Changelog.md.

[codecov]

Codecov Report

❌ Patch coverage is 55.73770% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.35%. Comparing base (c5affc5) to head (2cd4c99).
⚠️ Report is 69 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/modules/SchemaGraph/Sidebar/NodeLabelDetails.tsx	0.00%	5 Missing and 9 partials ⚠️
...ules/SchemaGraph/Sidebar/EdgeConnectionDetails.tsx	0.00%	6 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1533       +/-   ##
===========================================
+ Coverage   47.81%   65.35%   +17.53%     
===========================================
  Files         382      356       -26     
  Lines        8525     7960      -565     
  Branches     3159     2917      -242     
===========================================
+ Hits         4076     5202     +1126     
+ Misses       3070     1991     -1079     
+ Partials     1379      767      -612     

Flag	Coverage Δ
unittests	65.35% <55.73%> (+17.53%)	⬆️

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