Improve reflection-based method invocation with caching and error handling

[Copilot]

Addresses feedback on reflection-based method invocation in BuildSiteAsync that could fail at runtime with unclear errors and had performance overhead.

Performance

• Cache MethodInfo after first lookup using thread-safe double-checked locking
• Prevents repeated reflection queries on each build operation

Error Handling

• Extract method discovery into GetBuildAsyncMethod() with explicit validation:
  • No BuildAsync method found
  • Wrong signature (expected 5 generic params, 3 method params)
  • Multiple overloads (ambiguous)
• Preserve stack traces on TargetInvocationException using ExceptionDispatchInfo
• Wrap reflection failures with actionable error messages

Before:

var buildMethod = _generator!.GetType()
    .GetMethods()
    .Single(m => /* inline predicate */);
var task = (Task?)buildMethod.MakeGenericMethod(...).Invoke(...);
return task ?? Task.CompletedTask;

After:

if (_cachedBuildMethod == null)
{
    lock (_cacheLock)
    {
        if (_cachedBuildMethod == null)
        {
            _cachedBuildMethod = GetBuildAsyncMethod();
        }
    }
}
var closedMethod = _cachedBuildMethod.MakeGenericMethod(...);
// + comprehensive exception handling

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