Remove SSR buffering by blocking sync context queue during async writes

[javiercn]

Summary

This change removes the need for synchronous buffering when writing SSR render batches to the HTTP response. The challenge was that we needed to write render batches synchronously because yielding the thread while writing to the output allows other continuations to be scheduled and modify contents between async boundaries.

Solution

Instead of buffering content synchronously and then writing it, we now:

1. Move shared source files: RendererSynchronizationContext and RendererSynchronizationContextDispatcher are moved to src/Components/Shared/src/ so they can be compiled into both Components and Endpoints assemblies with different namespaces (via COMPONENTS_ENDPOINTS define).

2. Add EnqueueBlockingTask(Task) method: This method inserts a blocking task into the sync context queue, preventing other continuations from running until it completes.

3. EndpointHtmlRenderer creates its own dispatcher: This allows access to the Endpoints-specific RendererSynchronizationContext which has the EnqueueBlockingTask method.

4. Add WriteContentAsync helper: In RazorComponentEndpointInvoker, this helper:

   • Creates a TaskCompletionSource with RunContinuationsAsynchronously
   • Enqueues the TCS.Task to block the sync context queue
   • Writes content asynchronously with ConfigureAwait(false) to avoid deadlock
   • Completes the TCS to unblock the queue

Benefits

• Maintains thread safety (no other continuations can modify state while we write)
• Allows async I/O operations instead of synchronous buffering
• Reduces memory allocations from buffering

Testing

• All 480 Endpoints unit tests pass
• HandlesOverlappingStreamingBatches E2E test (added in Fix corrupted streaming SSR output #50380 to verify streaming SSR output corruption fix) passes