arrow

GH-41329: [C++][Gandiva] Fix gandiva cache size env var (#41330)

### Rationale for this change

Gandiva cache size validity checks are not robust enough (the negativity test is broken), and they are not currently tested.

### What changes are included in this PR?

1. Fix checking gandiva cache size env var.
2. Make cache size static so it only gets evaluated once.
3. Add test cases.
4. Enrich the description in the document about this env var.

### Are these changes tested?

UT included.

### Are there any user-facing changes?

None.

* GitHub Issue: #41329

Lead-authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Co-authored-by: Rossi Sun <zanmato1984@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>