fix: update ShapeStream mock constructor for vitest v4

Vitest v4 changed how mock functions work as constructors. Updated
the ShapeStream mock to use mockImplementation with a regular function
instead of an arrow function, allowing it to work properly with `new`.

Fixes electric-db-collection test failures caused by "not a constructor"
error in vitest v4.

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

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

Author: KyleAMathews

packages/electric-db-collection/tests/electric-live-query.test.ts

@@ -62,7 +62,9 @@ vi.mock(`@electric-sql/client`, async () => {
   const actual = await vi.importActual(`@electric-sql/client`)
   return {
     ...actual,
-    ShapeStream: vi.fn(() => mockStream),
+    ShapeStream: vi.fn().mockImplementation(function () {
+      return mockStream
+    }),
   }
 })
 

packages/electric-db-collection/tests/electric.test.ts

@@ -27,7 +27,9 @@ vi.mock(`@electric-sql/client`, async () => {
   const actual = await vi.importActual(`@electric-sql/client`)
   return {
     ...actual,
-    ShapeStream: vi.fn(() => mockStream),
+    ShapeStream: vi.fn().mockImplementation(function () {
+      return mockStream
+    }),
   }
 })