Mock generator predicate locals can collide with interface parameter names #7433

Summary

Mocking an interface whose parameter names collide with generated predicate-conversion locals can produce invalid C#. Generated locals need unique names so valid interfaces can be mocked.

Reproduction

Declare public interface IService { void Send(string value, string __fa_value); } and generate a mock with Mock.Of<IService>(). The generated predicate overload for value declares a local __fa_value in a method already containing a parameter with that name, producing CS0136.

Expected and actual

Expected: generated locals avoid all parameter names. Actual: EmitSingleFuncOverload prefixes the raw parameter name with __fa_ without checking the other parameters. This naming scheme predates #7415; its reviewer noticed the gap.

Environment

Generator source at c663d1a, src/TUnit.Mocks.SourceGenerator/Builders/MockMembersBuilder.cs, lines 1305–1309. Confirmed by inspecting emitted source construction; compilation reproduction has not been run.

Please allocate collision-free local names for predicate conversions and add a generator regression test.