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GH-38242: [Java] Fix incorrect internal struct accounting for DenseUnionVector#getBufferSizeFor (#38305)

### What changes are included in this PR?

Fix incorrect implementation of `DenseUnionVector.getBufferSizeFor`.
Sum the type id counts before calling `getBufferSizeFor` on the union's child vectors.

### Are these changes tested?

Yes. A test verifies the OOB read (requires bounds check), and an example count is correct, fails as expected before fix.

**This PR contains a "Critical Fix".** 

For users of DenseUnionVector the size can calculated incorrectly, as well as cause of out-of-bounds buffer reads which may return garbage (or potentially segfaulting) if bounds checking is off.

* Closes: #38242

Authored-by: Dan Stone <wotbrew@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>