Resolves a specific SQL field from PlatformVariants for the given platform. Returns the platform-specific variant if available, or null to signal fallback to base field.
The parsed PlatformVariantsJSON (can be null)
The field key in the PlatformVariantsJSON (e.g., 'SQL', 'FilterText')
The active database platform
The platform-specific SQL string, or null if no variant exists
Resolves a specific SQL field from PlatformVariants for the given platform. Returns the platform-specific variant if available, or null to signal fallback to base field.