Add support for internal types in validation source generator (#65632)

[XL1TTE]

Add support for internal types in validation source generator (#65632)

This change adds an opt-in mechanism for applications that keep request models internal for encapsulation purposes.
Added test coverage for internal types, internal properties, and verification that private/protected members remain excluded from validation.

Description

By default, the validation source generator only processes public types and members. When IncludeInternalTypes() call detected, ValidationsGenerator will start to include internal types and members to validation process.

Added Tests

Internal Types Validation

Test Name	Description	Expected Behavior
InternalType_NotValidated_ByDefault	Internal class with [ValidatableType] attribute without configuration	Internal type is not discovered for validation by default
InternalType_Validated_With_IncludeInternalTypes	Internal class with [ValidatableType] attribute with IncludeInternalTypes()	Internal type is discovered and validated when option is enabled

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Internal Properties Validation

Test Name	Description	Expected Behavior
InternalProperty_NotValidated_ByDefault	Public class with internal property marked [Required]	Internal property is not validated by default
InternalProperty_Validated_With_IncludeInternalTypes	Public class with internal property marked [Required] with IncludeInternalTypes()	Internal property is validated when option is enabled

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Private and Protected Properties Validation

Test Name	Description	Expected Behavior
PrivateProperty_Never_Validated	Public class with private property marked [Required] with IncludeInternalTypes()	Private property is never validated (even with IncludeInternalTypes())
ProtectedProperty_Never_Validated	Public class with protected property marked [Required] with IncludeInternalTypes()	Protected property is never validated (even with IncludeInternalTypes())

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Internal Nested Types Validation

Test Name	Description	Expected Behavior
InternalNestedType_Validated_With_IncludeInternalType	Public class with internal nested type property with IncludeInternalTypes()	Internal nested type is validated when option is enabled

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Configuration Syntax Validation

Test Name	Description	Expected Behavior
IncludeInternalTypes_BlockLambda_Supported	Configuration using block lambda syntax { options.IncludeInternalTypes(); }	Block lambda syntax works correctly
IncludeInternalTypes_ExpressionLambda_Supported	Configuration using expression lambda syntax options => options.IncludeInternalTypes()	Expression lambda syntax works correctly

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Test Summary

• Total Tests Added: 9
• Key Assertions:
  • Default behavior excludes internal types and properties
  • IncludeInternalTypes() enables internal types and properties
  • Private and protected members remain excluded regardless of configuration
  • Both lambda syntax variations are supported

Usage:

builder.Services.AddValidation(options => options.IncludeInternalTypes());

Fixes #65632

[XL1TTE]

@dotnet-policy-service agree