A null ParameterTemplate in Parameters/PositionalParameters throws NullReferenceException after the declaration is half-written: void M} / record R #198

What's wrong

ClassTemplate.Validate() runs before anything is written, so that a bad member cannot leave partial output (#164) (ClassTemplate.cs#L105-L118). It never checks parameter lists, though. TemplateRendering.WriteParameterList (TemplateRendering.cs#L199-L209) dereferences each entry partway through writing:

foreach (ParameterTemplate parameterTemplate in parameters)
{
	parameterStrings.Add(RenderFragment(codeBlocker, parameterTemplate.WriteTo));

Collection<ParameterTemplate> accepts null. A null entry therefore throws after the declaration has started, and the class body's using Scope then writes its closing brace straight onto the half-written declaration.

Reproduction

var m = new MethodTemplate { Name = "M", Type = "void", BodyFactory = _ => { } };
m.Parameters.Add(null!);
cb.AddClass(new ClassTemplate { Name = "C", Members = { m } });

This throws NullReferenceException and leaves class C\n{\n\tvoid M}\n in the writer.

var r = new ClassTemplate { Name = "R", Kind = TypeKind.Record };
r.PositionalParameters.Add(null!);
cb.AddClass(r);

This throws NullReferenceException and leaves record R in the writer.

A null DocTag in a member's Documentation.Params behaves the same way. It leaves a closed class that has silently lost the member, which is the #164 symptom; that case overlaps with #188. A null entry in Members or NestedClasses does throw before anything is written, but it throws NullReferenceException. By contrast, AddClasses and AddSourceFile throw ArgumentNullException for a null type.

Why it matters

The library's contract, stated in the remarks on TemplateBase.Validate and in #164/#169, is that a template it refuses leaves nothing behind. Here the caller gets partial output plus an NRE that doesn't name the template.

Suggested fix / acceptance criteria

• In Validate (class, method, constructor, operator), reject null entries in Members, NestedClasses, Parameters, PositionalParameters and the documentation tag lists with ArgumentException/ArgumentNullException that names the template, before anything is written.
• Add tests asserting the writer is empty after the exception, for a null method parameter and for a null positional parameter.

Related but distinct: #176 (a throwing BodyFactory leaves } on a half-written member) and #188 (an invalid doc comment).