Add scalar FixedTimeEquals overload

[vcsjones]

Fixes #127584

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[Copilot]

Pull request overview

Adds a new scalar overload for CryptographicOperations.FixedTimeEquals so callers can compare a buffer against a repeated byte value without materializing a second span. In this codebase, that primarily simplifies constant-time zero-buffer checks such as the Windows X25519 shared-secret validation path.

Changes:

• Added CryptographicOperations.FixedTimeEquals(ReadOnlySpan<byte> source, byte value) to the implementation and ref assembly.
• Replaced the Windows X25519 all-zero shared-secret check to use the new scalar overload instead of a hard-coded zero span.
• Expanded FixedTimeEquals tests to cover scalar true/false cases and verify both overloads retain the required method implementation flags.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/tests/FixedTimeEqualsTests.cs	Adds scalar overload coverage and updates the method-attribute reflection test.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanImplementation.Windows.cs	Switches the all-zero shared-secret check to the new scalar overload.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/CryptographicOperations.cs	Implements and documents the new public scalar overload.
src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs	Exposes the new public API in the ref assembly.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 0

[vcsjones]

/ba-g failures are unrelated