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

src/plugin-sdk/subpaths.test.ts

@@ -84,8 +84,9 @@ describe("plugin-sdk subpath exports", () => {
   });
 
   it("exports WhatsApp helpers", () => {
-    expect(typeof whatsappSdk.resolveWhatsAppAccount).toBe("function");
-    expect(typeof whatsappSdk.whatsappOnboardingAdapter).toBe("object");
+    // WhatsApp-specific functions (resolveWhatsAppAccount, whatsappOnboardingAdapter) moved to extensions/whatsapp/src/
+    expect(typeof whatsappSdk.WhatsAppConfigSchema).toBe("object");
+    expect(typeof whatsappSdk.resolveWhatsAppOutboundTarget).toBe("function");
   });
 
   it("exports LINE helpers", () => {