Implement entire method generation (without prior partial declaration)

[Copilot]

Adds support for generating complete methods via Generate.Method() fluent API, where the method name, return type, parameters, and body are all specified in the generator — no partial method declaration required.

public partial class ColorsClassFluentCreated
{
    [MethodBodyGenerator("GetAllColorsString")]
    static IMethodBodyGenerator GetAllColorsString_Generator() =>
        Generate.Method()
            .WithName("GetAllColorsString")
            .WithReturnType<string>()
            .WithNoParameters()
            .BodyReturningConstant(() => string.Join(", ", Enum.GetNames<ColorsEnum>()));
}

// Generates:
// public string GetAllColorsString() { return "Red, Green, Blue"; }

Abstractions

• Completed IMethodBuilderStage1–Stage4 interfaces with terminal methods (BodyReturningConstant, BodyWithSwitchStatement, UseProvidedBody)
• Added IGeneratorsFactory.StartFluentApiBuilderForMethod() and supporting IMethodBodyGeneratorStage0, IMethodBodyBuilder interfaces
• Reverted switch body ForCases to params object[] and ReturnConstantValue to return IMethodBodyGeneratorSwitchBody (restoring chained ForCases support); removed intermediate CaseStage2

Generator pipeline

• Made PartialMethod nullable in GeneratesMethodGenerationTarget; collector now allows missing partial methods for fluent patterns
• Added GenerateFromEntireMethodPattern pipeline path and ExecuteEntireMethodGeneratorMethod runtime method
• BuildDummyImplementation now scans for [MethodBodyGenerator] targets without partial methods and synthesizes dummy declarations (extracting signature from the fluent API syntax), so other generators' execution compilations don't fail

Recording factory

• Added MethodRecord and recording implementations for all Method builder stages
• RecordingGeneratorsFactory implements IGeneratorsFactory and captures method name, return type, parameter types, constant values, and switch body data

Examples & tests

• Added ColorsClassFluentCreated (constant return, no params) and MapperFluentCreated (switch with enum mapping) examples
• Added corresponding integration tests verifying runtime output and generated code shape
• Updated existing tests for renamed interfaces (UseBody → UseProvidedBody, signature simplification)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement entire method generation</issue_title>
<issue_description>So far, only the body code generation has been implemented.
I've added some abstractions and examples to allow for entire method generation (without prior partial declaration).

Implement it.

Take a look at those examples. Don't modify them, just make sure they compile, add automated tests and implement them.

ColorsClassFluentCreated
MapperFluentCreated

For now forget about Method Template. Focus only on entire method generation (not only body).

The solutions needs to compile and tests need to pass after this PR. Examples should not be modified.</issue_description>

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

• Fixes Implement entire method generation #84

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