Add MSGH008 return type validation for MethodBodyGenerator

[Copilot]

[MethodBodyGenerator] methods with invalid return types were silently accepted. The defined MSGH008 diagnostic was never reported.

• Added validation in GeneratesMethodGenerationTargetCollector.Collect() after partial method resolution: generator return type must be either IMethodBodyGenerator (fluent pattern) or match the target partial method's return type (simple pattern)
• Diagnostic location uses generatorMethod.ReturnType.GetLocation() to highlight exactly the return type token

// Returns object instead of int — now reports MSGH008 on "object"
[MethodBodyGenerator(nameof(JustReturnConstant))]
public static object JustReturnConstantGenerator() =>
    Generate.MethodBody()
        .ForMethod().WithReturnType<int>().WithNoParameters()
        .UseProvidedBody(() => 42);

All 3 WrongReturnType tests now pass. Full suite green with no regressions.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Write code to make WrongReturnType tests pass</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Write code to make WrongReturnType tests pass #91

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[Copilot]

Pull request overview

This PR fixes missing reporting of the already-defined MSGH008 diagnostic by adding return-type validation for [MethodBodyGenerator] methods during generation target collection, ensuring invalid generator return types are rejected early and highlighted precisely at the return-type syntax.

Changes:

• Validate generator method return type after resolving the target partial method.
• Report MSGH008 when the generator return type is neither IMethodBodyGenerator (fluent pattern) nor the target partial method’s return type (simple pattern).
• Use the generator method’s return type syntax location so the diagnostic highlights only the return type token.

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