Refactor fluent generator API to explicit Method().WithParameter().WithReturnType() chain

[Copilot]

This PR replaces the ambiguous Generate.MethodImplementation<...>() entrypoint with an explicit fluent chain that makes parameter vs return type intent clear. It also updates all in-repo fluent examples/tests and removes the old API surface (no backward-compatibility shim, per issue scope).

• Fluent API surface refactor (Abstractions)

  • Removed Generate.MethodImplementation(), Generate.MethodImplementation<TReturnType>(), and Generate.MethodImplementation<TArg1, TReturnType>().
  • Added new builder-based entrypoint:
    • Generate.Method()
    • WithParameter<TArg1>()
    • WithReturnType<TReturnType>()
  • Introduced minimal builder contracts/impls to route to existing IGeneratorsFactory.CreateImplementation(...) methods without changing generator internals.

• Usage migration in examples

  • Updated PiExampleFluent to:
    • Generate.Method().WithParameter<int>().WithReturnType<int>()
  • Updated ColorsClassFluent to:
    • Generate.Method().WithReturnType<string>()

• Usage migration + coverage in tests

  • Updated existing fluent test generator usage to the new chain.
  • Added mapper-like fluent coverage demonstrating non-identical parameter/return intent (int -> string) and validating generated switch output shape.

[GeneratesMethod(nameof(MapToMammal))]
static IMethodImplementationGenerator MapToMammal_Generator() =>
    Generate
        .Method().WithParameter<int>().WithReturnType<string>()
        .WithSwitchBody()
        .ForCases(1).ReturnConstantValue(_ => "Dog")
        .ForCases(2).ReturnConstantValue(_ => "Cat")
        .ForDefaultCase().WithBody(_ => () => "Unknown");

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fluent API Refactor</issue_title>
<issue_description>In PiExampleFluent the fluent code starts like this:

        Generate
            .MethodImplementation<int, int>()

However, this is not intuitive. You can't tell which generic type argument is the return type and which is the argument. Thats why, in the new Mapper class there is a new flue API example:

 Generate
            .Method().WithParameter<FourLeggedAnimal>().WithReturnType<MammalAnimal>()

Implement it. Replace the old way with the new way (both here, other examples, and in tests).
Remove the old way, the package was not released yet so no need to ensure backwards compatiblity nor Deprecated attribute. Just remove the old way and relpace all usages with the new way.

Also, ensure the new example works, create something similar for tests.</issue_description>

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

• Fixes Fluent API Refactor #17

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[Copilot]

Pull request overview

This PR refactors the fluent generator entrypoint from ambiguous Generate.MethodImplementation<...>() overloads to an explicit builder chain (Generate.Method().WithParameter<T>().WithReturnType<T>()) to make parameter vs return type intent clear, and migrates in-repo examples/tests accordingly.

Changes:

• Replaced Generate.MethodImplementation* overloads with a builder-based fluent entrypoint (Generate.Method() + WithParameter/WithReturnType).
• Migrated fluent usage in examples to the new chain.
• Updated tests to the new chain and added mapper-like coverage (int -> string) validating both runtime output and generated code shape.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
MattSourceGenHelpers.Abstractions/Generate.cs	Removes old entrypoints and adds the new method builder chain routing to IGeneratorsFactory.CreateImplementation(...).
MattSourceGenHelpers.Examples/PiExampleFluent.cs	Migrates Pi fluent example to Method().WithParameter<int>().WithReturnType<int>().
MattSourceGenHelpers.Examples/ColorsClassFluent.cs	Migrates Colors fluent example to Method().WithReturnType<string>().
MattSourceGenHelpers.Tests/PiExampleFluentTests.cs	Updates fluent usage to the new chain and adds mapper-like tests for int -> string generation/output.

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[Copilot]

MethodBuilder / MethodBuilder<TArg1> are only constructed internally via Generate.Method() and are not referenced elsewhere in the repo. Consider making these concrete builder types internal (and potentially sealed) to avoid expanding the public API surface unnecessarily; consumers can still program against the public IMethodBuilder interfaces returned by Generate.Method().

[Copilot]

The Generate.Method().WithReturnType<TReturnType>() (no-parameter) path is part of the new fluent entrypoint, but current tests only exercise the .WithParameter<TArg1>().WithReturnType<TReturnType>() chain. Adding at least one test that uses the parameterless chain (similar to ColorsClassFluent) would help catch regressions in this new API surface.