Rename ValidationRouteHandlerBuilderExtensions.cs to ValidationEndpoi…

[fkucukkara]

…ntConventionBuilderExtensions.cs for consistency

Fix file name inconsistency for ValidationEndpointConventionBuilderExtensions

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Summary of the changes (Less than 80 chars)

Description

No functional changes were made—the class and its methods remain identical. This ensures better maintainability and adherence to naming conventions. The associated tests and samples continue to work as expected.

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