refactor: move WhatsApp channel implementation to extensions/ (#45725)

* refactor: move WhatsApp channel from src/web/ to extensions/whatsapp/

Move all WhatsApp implementation code (77 source/test files + 9 channel
plugin files) from src/web/ and src/channels/plugins/*/whatsapp* to
extensions/whatsapp/src/.

- Leave thin re-export shims at all original locations so cross-cutting
  imports continue to resolve
- Update plugin-sdk/whatsapp.ts to only re-export generic framework
  utilities; channel-specific functions imported locally by the extension
- Update vi.mock paths in 15 cross-cutting test files
- Rename outbound.ts -> send.ts to match extension naming conventions
  and avoid false positive in cfg-threading guard test
- Widen tsconfig.plugin-sdk.dts.json rootDir to support shim->extension
  cross-directory references

Part of the core-channels-to-extensions migration (PR 6/10).

* style: format WhatsApp extension files

* fix: correct stale import paths in WhatsApp extension tests

Fix vi.importActual, test mock, and hardcoded source paths that weren't
updated during the file move:
- media.test.ts: vi.importActual path
- onboarding.test.ts: vi.importActual path
- test-helpers.ts: test/mocks/baileys.js path
- monitor-inbox.test-harness.ts: incomplete media/store mock
- login.test.ts: hardcoded source file path
- message-action-runner.media.test.ts: vi.mock/importActual path

scripts/write-plugin-sdk-entry-dts.ts

@@ -1,8 +1,8 @@
 import fs from "node:fs";
 import path from "node:path";
 
-// `tsc` emits declarations under `dist/plugin-sdk/plugin-sdk/*` because the source lives
-// at `src/plugin-sdk/*` and `rootDir` is `src/`.
+// `tsc` emits declarations under `dist/plugin-sdk/src/plugin-sdk/*` because the source lives
+// at `src/plugin-sdk/*` and `rootDir` is `.` (repo root, to support cross-src/extensions refs).
 //
 // Our package export map points subpath `types` at `dist/plugin-sdk/<entry>.d.ts`, so we
 // generate stable entry d.ts files that re-export the real declarations.
@@ -56,5 +56,5 @@ for (const entry of entrypoints) {
   const out = path.join(process.cwd(), `dist/plugin-sdk/${entry}.d.ts`);
   fs.mkdirSync(path.dirname(out), { recursive: true });
   // NodeNext: reference the runtime specifier with `.js`, TS will map it to `.d.ts`.
-  fs.writeFileSync(out, `export * from "./plugin-sdk/${entry}.js";\n`, "utf8");
+  fs.writeFileSync(out, `export * from "./src/plugin-sdk/${entry}.js";\n`, "utf8");
 }