fix: Keep IParallelLimitProvider and GetLock internal

Address PR review feedback - the visibility change from internal to
public was accidental. IParallelLimitProvider is internal infrastructure
and should not be part of the public API surface.

- Revert IParallelLimitProvider from public to internal
- Change ParallelLimiterAttribute.GetLock to internal

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: thomhurst

src/ModularPipelines/Attributes/ParallelLimiterAttribute.cs

@@ -17,7 +17,7 @@ public ParallelLimiterAttribute() : base(typeof(TParallelLimit))
     }
 
     /// <inheritdoc />
-    public override AsyncSemaphore GetLock(IParallelLimitProvider provider)
+    internal override AsyncSemaphore GetLock(IParallelLimitProvider provider)
     {
         return provider.GetLock<TParallelLimit>();
     }
@@ -52,5 +52,5 @@ protected ParallelLimiterAttribute(Type type)
     /// </summary>
     /// <param name="provider">The parallel limit provider.</param>
     /// <returns>The semaphore for this limit type.</returns>
-    public abstract AsyncSemaphore GetLock(IParallelLimitProvider provider);
+    internal abstract AsyncSemaphore GetLock(IParallelLimitProvider provider);
 }

src/ModularPipelines/Helpers/IParallelLimitProvider.cs

@@ -4,7 +4,7 @@
 
 namespace ModularPipelines.Helpers;
 
-public interface IParallelLimitProvider
+internal interface IParallelLimitProvider
 {
     /// <summary>
     /// Gets a semaphore lock for the specified parallel limit type.