C#: Fix template substitution for method names and variadic args

[knutwannheden]

Problem

CSharpTemplate.Apply() outputs placeholder names instead of captured values when captures appear in method-name or variadic-argument positions:

var method = Capture.Of<Identifier>("method");
var args = Capture.Variadic<Expression>("args");

var pat = CSharpPattern.Create($"new Random().{method}({args})");
var tmpl = CSharpTemplate.Create($"Random.Shared.{method}({args})");

When matching new Random().Next(10), the template produces:

Random.Shared.__plh_method__(__plh_args__)

instead of:

Random.Shared.Next(10)

Summary

• Fix SubstitutionVisitor to substitute placeholders in method-name positions (MethodInvocation.Name) and field-name positions (FieldAccess.Name), which the base visitor never visits
• Fix variadic argument expansion: detect variadic capture placeholders in argument lists and replace them with the captured expression list
• Fix MatchResult.GetList<T> to handle IReadOnlyList<object> (as stored by the pattern matcher) via Cast<T>() instead of failing the type check silently
• Handle empty variadic captures by removing the placeholder argument rather than leaving it in the output
• Use Cast<T>() instead of OfType<T>() to fail fast on type mismatches

Test plan

• SubstitutesMethodNameCapture: pattern captures method name, template substitutes it into a different target
• SubstitutesVariadicArgs: variadic capture of multiple args, template substitutes into different method
• SubstitutesMethodNameAndVariadicArgs: combined method name + variadic args (new Random().Next(10) → Random.Shared.Next(10))
• SubstitutesFieldNameCapture: isolated field-name substitution (DateTime.Now → DateTimeOffset.Now)
• All 95 existing template tests continue to pass