[RutulPatel8]

Implements the two ReadOnlySpan<T>.CommonSuffixLength overloads approved in #127630.

CommonPrefixLength counts matching elements from the start of two spans. This API counts matching elements from the end, including when the spans have different lengths. It supports default equality or a supplied IEqualityComparer<T>.

The change adds the implementation, public reference signatures, and tests for empty and unequal spans, custom comparers, reference types, and floating-point equality.

Validation:

• Managed CoreLib, System.Memory reference assembly, shared libraries, and System.Memory test project built with no warnings or errors.
• The targeted tests could not run locally because the native test host lacks dotnet.exe; this machine needs the Visual Studio C++ tools to build that host.

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[RutulPatel8]

/azp run runtime

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[RutulPatel8]

@dotnet/area-system-memory Could an area maintainer review this contribution once the refreshed CI run completes? The previous run failed in tests outside System.Memory. The Azure bot rejected my manual rerun command because fork authors lack permission, so I pushed an empty commit (165bf6ccc795) to trigger fresh CI without changing the implementation. I also noticed the overlapping #133453; guidance on which PR you would prefer to take would be appreciated. Thanks!