[C++] Harden synthetic OOM tests for mimalloc

[k8ika0s]

Closes #49280

Summary

This PR makes synthetic OOM unit tests allocator-stable when mimalloc is active.

It updates:

• cpp/src/arrow/buffer_test.cc
• cpp/src/arrow/memory_pool_test.h

Problem

These tests intentionally request extremely large allocations to force OutOfMemory.
Using near-max bounds can hit allocator-specific fatal behavior (especially with mimalloc)
before Arrow returns a normal OOM status, making tests environment-dependent.

What changed

1. In ResizeOOM and TestOOM, detect mimalloc backend and GTEST_SKIP() synthetic OOM assertions.
2. Clamp synthetic huge allocation upper bound to 1 << 48 (256 TB), which is still impossible in practice but avoids extreme allocator boundary paths.
3. Keep existing alignment-overflow guard (-63) and ASSERT_RAISES(OutOfMemory, ...) logic for non-mimalloc backends.

Why this approach

• Scope is intentionally minimal and test-only.
• Preserves the goal of OOM-path coverage where behavior is reliable.
• Removes allocator-fatal coupling from CI/unit-test expectations.

Risk / Compatibility

• No runtime library behavior change.
• No API/ABI impact.
• Test behavior only.

[github-actions]

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• Other pull requests
• Contribution Guidelines - Contributing Overview