[I] [C++] Synthetic OOM tests are allocator-sensitive with mimalloc [arrow]

["k8ika0s (via GitHub)" <gi...@apache.org>]

k8ika0s opened a new issue, #49280:
URL: https://github.com/apache/arrow/issues/49280

   ### Describe the bug, including details regarding any error messages, version, and platform.
   
   ### Describe the bug
   
   Two C++ unit-test paths use synthetic "impossibly large" allocations to assert `OutOfMemory`:
   
   - `TypedTestBuffer.ResizeOOM` in `/Users/kaitlyndavis/dev/github.com/k8ika0s/arrow_s390x/cpp/src/arrow/buffer_test.cc`
   - `TestMemoryPoolBase::TestOOM` (used by `arrow-misc-test`) in `/Users/kaitlyndavis/dev/github.com/k8ika0s/arrow_s390x/cpp/src/arrow/memory_pool_test.h`
   
   On some allocator/platform combinations (notably with `mimalloc` enabled), the near-`max` request can trigger allocator fatal behavior before Arrow can surface `Status::OutOfMemory`. This makes the test behavior allocator-dependent and can cause instability in CI/dev runs.
   
   ### To Reproduce
   
   Build C++ tests with mimalloc enabled and run OOM-focused cases, for example:
   
   ```bash
   cmake -S cpp -B build -DARROW_BUILD_TESTS=ON -DARROW_MIMALLOC=ON -DARROW_JEMALLOC=OFF
   cmake --build build --target arrow-buffer-test arrow-misc-test -j
   ./build/debug/arrow-buffer-test --gtest_filter='*ResizeOOM*'
   ./build/debug/arrow-misc-test --gtest_filter='*OOM*'
   ```
   
   ### Expected behavior
   
   Synthetic OOM tests should be deterministic and should not depend on allocator internals/fatal paths.
   
   ### Actual behavior
   
   With mimalloc, the "max-size" synthetic OOM request may not reliably produce Arrow OutOfMemory; behavior can depend on allocator failure mode.
   
   ### Proposed fix
   - Keep synthetic OOM tests, but clamp requests to a still-impossible size (e.g. 1 << 48, 256 TB) instead of raw near-max bounds.
   - Skip these synthetic OOM checks under mimalloc backend where allocator fatal path can bypass Arrow error propagation.
   
   This is test-only hardening (no production runtime code changes).
   
   ### Component(s)
   
   C++


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