Fix: handle duplicate field names in struct array cast

[connortsui20]

Technically fixes #5865

TODO description.

To be honest, I really dislike this change. It doesn't make sense even to me.

See #6010

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.95%. Comparing base (a30298b) to head (80f27dd).

Files with missing lines	Patch %	Lines
vortex-array/src/arrays/struct_/compute/cast.rs	97.22%	1 Missing ⚠️

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