fix: apply custom Map results when mapping to existing target (#900)

[leno23]

Summary

• Fixes MapToTarget (mapper.Map(dto, existingEntity)) ignoring custom .Map() resolvers when the destination member already has a value.
• When a destination member has an explicit custom .Map() configuration, skip passing the existing destination member into nested adaptation so the resolver result is assigned directly.
• Adds regression tests reproducing issue Mapping an object with getter only properties seems to be broken in v10.0.6 (works as expected in 7.4.0) #900 for MapToTarget with pre-existing destination values, null destination members, and new-object mapping.

Root cause

For MapToTarget, ClassAdapter always passed the existing destination member expression into CreateAdaptExpression. That triggered nested MapToTarget adaptation for the member type. For immutable/getter-only types (like primary-constructor classes), nested mapping preserves the existing destination instance instead of replacing it with the custom .Map() result.

Test plan

• CI passes on Mapster.Tests
• WhenMappingGetterOnlyMapToTargetRegression.MapToTarget_WithCustomMap_ShouldReplaceExistingDestinationMemberValue
• Existing MapToTarget regression tests remain green

Fixes #900

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