fix: prevent delegation to inexact user mappings

[AmirTahan80]

Fixes #1793.

Summary

Mapperly could reuse a user-defined mapping through BuildDelegatedMapping even when the requested source and target types did not match the user mapping exactly. For example, a mapping returning Target[] could be selected when Mapperly needed IEnumerable<Target>.

Root cause

MappingBuilderContext.BuildDelegatedMapping accepted any existing mapping found for compatible types. This also included IUserMapping implementations, although user-defined mappings should only participate when their source and target types match exactly.

Changes

• Exclude IUserMapping instances from generalized delegated mapping resolution.
• Continue allowing Mapperly-generated mappings to be reused through delegation.
• Add a regression test based on the reproduction from Incorrect warning RMG020 for nested types when custom method return types do not match target type exactly #1793.

Impact

Mapperly now generates the appropriate collection and element mappings instead of selecting an inexact user-defined mapping. User mappings continue to work when their source and target types match exactly.

This changes the behavior of inexact user mappings and is therefore appropriate for the next major release, as discussed in the issue.

Validation

• dotnet build Riok.Mapperly.slnx /p:TreatWarningsAsErrors=true
• dotnet test Riok.Mapperly.slnx --no-build --no-restore
• 1,726 tests passed, 2 skipped, 0 failed
• CSharpier, style, and analyzer checks passed

[AmirTahan80]

Additional implementation context: this follows the approach proposed in #1793 by excluding IUserMapping instances from generalized delegation in MappingBuilderContext.BuildDelegatedMapping. Mapperly-generated mappings remain eligible for delegation.

The regression test uses the reported ICollection<Source?> to IEnumerable<Target> scenario and verifies that Mapperly generates and calls MapToTargetArray instead of selecting the inexact user-defined CustomMapToTargetArray method.

Validation completed locally:

• Build succeeded with warnings treated as errors
• 1,726 tests passed, 2 skipped, 0 failed
• CSharpier, style, and analyzer checks passed