refactor(plugin-entry): move createPluginModule to testing module

createPluginModule and PluginModuleDeps were exposed at package entry as
a test seam. Their export creates accidental public TS API obligations
for internal manager/tool/hook constructor types. Move to
src/testing/create-plugin-module.ts so only tests reach them.

Closes HIGH-8

Co-authored-by: api-surface (deep / gpt-5.3-codex high)

src/index.telemetry.test.ts

@@ -1,5 +1,5 @@
 import { beforeEach, describe, expect, it, mock } from "bun:test"
-import { createPluginModule } from "./index"
+import { createPluginModule } from "./testing/create-plugin-module"
 
 const mockInitConfigContext = mock(() => {})
 const mockInjectServerAuthIntoClient = mock(() => {})