Add tests for API key auth timing behavior

[hman38705]

Area: ApiKeyAuth::verify

Problem: Linear equality checks may leak timing differences.

Testing gap: No tests/benchmarks for constant-time comparison policy.

Acceptance tests: Adopt constant-time compare and add regression test for behavior.

[Ryjen1]

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Kindly assign to me

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[techisigu]

@techisigu has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I’m a smart contract developer who turns complex ideas into secure, production-ready blockchain code.
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