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GH-41198: [C#] Fix concatenation of union arrays (#41226)

### Rationale for this change

Fixes concatenation of union arrays.

### What changes are included in this PR?

* Re-enables union array concatenation tests that were disabled in #41197 after making union array comparisons more thorough in the `ArrowReaderVerifier`
* Updates the union array concatenation logic to account for array lengths when concatenating the type and offset buffers, and fixes how the base offset is calculated.
* Fixes creating the type buffers for the array concatenation tests.

### Are these changes tested?

Yes, this uses the existing `ArrowArrayConcatenatorTests` tests.

### Are there any user-facing changes?

Yes, this is a user-facing bug fix.
* GitHub Issue: #41198

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>