#7625 Fix static typing for set and frozenset array vars

Summary

Fix static typing for set and frozenset array vars so literal creation preserves container types and indexing returns the
appropriate element var type.

• Widen array type bounds, literal creation overloads, and indexing annotations.
• Preserve array return types for array operations.
• Remove the frozenset test cast and add typed regression coverage.
• Add a changelog fragment.

Sequence component-prop runtime behavior is unchanged.

All Submissions

• Have you followed the guidelines stated in CONTRIBUTING.md?
• Have you checked for other open pull requests addressing this change?

Type of change

• Bug fix (non-breaking change which fixes an issue)

Changes To Core Features

• Added an explanation of the changes and why they should be included.
• Added regression tests.
• Successfully ran tests with the changes locally.

Validation

• Full unit suite: 12,107 passed, 26 skipped.
• Coverage: 80.18% (required: 72%).
• Linux-targeted Pyright: 0 errors, 1 warning in an unchanged file.
• All other pre-commit hooks passed.
• Native macOS Pyright reports 6 errors in unchanged benchmark files involving CPU-affinity APIs.
• Integration tests were not run.

Related to ENG-13480; surfaced in the review of #7622.

Closes #7623