GH-48094: [C++] Restrict SecureString capacity tail check to Linux

[ArnavBalyan]

Rationale for this change

• The test reads the unused tail of string past size(), which is undefined. libstdc++ keeps that range addressable, but MSVC does not.
• Ensure the check is not run for MSVC. Existing assertions still run, the unused capacity is not asserted on non linux platforms.
• Closes [C++] Arrow test 'arrow-utility-test' contains heap-buffer-overflow error #48094.

What changes are included in this PR?

• Guard for platform specific behavior.

Are these changes tested?

• Yes

Are there any user-facing changes?

• Yes

• GitHub Issue: [C++] Arrow test 'arrow-utility-test' contains heap-buffer-overflow error #48094

[github-actions]

⚠️ GitHub issue #48094 has been automatically assigned in GitHub to PR creator.

[ArnavBalyan]

cc @pitrou thanks

[pitrou]

Can we condition this on MSVC rather than Linux? The test can still be used under macOS and other platforms.

[ArnavBalyan]

done thanks

[pitrou]

+1, thank you @ArnavBalyan

[pitrou]

Oops, I should have changed the PR title before merging. Too late.

[conbench-apache-arrow]

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