feat: add dev warning for missing field definition labels in attribute-editor

[Who-is-PS]

Description

Adds a console warning via warnOnce when an AttributeEditor field definition is missing a label prop. The label is used as aria-label for the inputs in the column, so omitting it degrades the screen reader experience.

This change helps guide developers toward providing accessible labels without introducing a breaking change (making label required).

Related links, issue #, if available: AWSUI-61432

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (b0bfa53) to head (93e44b6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4337   +/-   ##
=======================================
  Coverage   97.43%   97.43%           
=======================================
  Files         897      897           
  Lines       26345    26352    +7     
  Branches     9517     9522    +5     
=======================================
+ Hits        25670    25677    +7     
+ Misses        669      632   -37     
- Partials        6       43   +37     

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