vfs: improve test coverage and add c8 ignore for SEA paths

Add comprehensive tests for URL arguments, throwIfNoEntry branches,
VirtualDir async methods, VirtualWriteStream error paths, Provider
base class, MemoryProvider edge cases, and VFS callback operations.
Add c8 ignore comments for SEA provider and SEA VFS initialization
code that requires actual SEA binary execution to test.

Author: mcollina

lib/internal/vfs/providers/sea.js

@@ -44,6 +44,7 @@ const kAssets = Symbol('kAssets');
 const kDirectories = Symbol('kDirectories');
 const kGetAsset = Symbol('kGetAsset');
 
+/* c8 ignore start -- SEA provider requires actual SEA binary execution */
 /**
  * File handle for SEA assets (read-only).
  */
@@ -436,6 +437,8 @@ class SEAProvider extends VirtualProvider {
   }
 }
 
+/* c8 ignore stop */
+
 module.exports = {
   SEAProvider,
 };

lib/internal/vfs/sea.js

@@ -20,6 +20,7 @@ const lazySEAProvider = getLazy(
   () => require('internal/vfs/providers/sea').SEAProvider,
 );
 
+/* c8 ignore start -- SEA VFS initialization requires actual SEA binary execution */
 /**
  * Creates a VirtualFileSystem populated with SEA assets using the Provider architecture.
  * Assets are mounted at the specified prefix (default: '/sea').
@@ -87,6 +88,8 @@ function getSeaVfs() {
   return cachedSeaVfs;
 }
 
+/* c8 ignore stop */
+
 module.exports = {
   createSeaVfs,
   initSeaVfs,