Fix config source gen with Dictionary with nullable Enum value

[tarekgh]

Fixes #115343

[Copilot]

Pull Request Overview

This PR addresses the issue with configuration source generation for dictionaries that have nullable enum values. The changes include adding a new test to verify this behavior, introducing supporting test classes and enums, and updating the code generation logic to unwrap nullable types correctly.

• Added new test case in ConfigurationBinderTests.cs to verify dictionary binding with nullable enum values
• Introduced OptionsWithDictionaryWithNullableEnumValue and MyValue enum in the test classes
• Modified CoreBindingHelpers.cs to unwrap NullableSpec for dictionary element types

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs	Added test for dictionary binding with nullable enum values
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs	Added supporting test classes and enum for nullable enum dictionary values
src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Emitter/CoreBindingHelpers.cs	Updated to unwrap nullable types for proper binding

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[ericstj]

LGTM, thanks for considering my feedback