GH-40128: [C++][Python] Preserve dictionaries in empty casts

[LarryHu0217]

What changed

• Run dictionary-to-dictionary casts through the cast kernel for zero-length inputs when the dictionary type changes.
• Add a regression test that verifies both the widened index type and preserved dictionary values.

The generic zero-length scalar fast path creates an empty output dictionary and discards categories that are still attached to the input array. Other zero-length scalar operations continue to use the existing fast path.

Testing

• cpp/build-40128-auto/debug/arrow-compute-scalar-cast-test (109 tests passed)
• clang-format --dry-run --Werror on both changed files
• git diff --check

AI assistance

I used Codex to help inspect the executor path, prepare the focused change, and draft the regression test. I reviewed the final diff, reproduced the failure before the fix, and ran the tests above locally.

Closes #40128

• GitHub Issue: [Python] Casting an empty dictionary array loses category information #40128

[uros-b]

Fix looks narrowly scoped and properly tested, thank you @LarryHu0217!