refactor(libs): consolidate messaging-internal into messaging library (#16386)

This change eliminates the circular dependency between messaging and messaging-internal libraries by merging them into a single messaging library.

Previously, messaging-internal imported from @bitwarden/messaging while messaging tried to import from @bitwarden/messaging-internal, creating an unresolvable circular dependency. This also violated Nx best practices by using cross-library file includes in tsconfig.lib.json.

Changes made:
- Moved all messaging-internal code (SubjectMessageSender, helpers, tests) into libs/messaging/src/
- Updated all imports to use relative paths instead of @bitwarden/messaging imports
- Removed the entire messaging-internal library and its configuration files
- Updated external references in apps/browser to import from @bitwarden/messaging
- Fixed libs/messaging/tsconfig.lib.json to use standard src/**/*.ts pattern
- Updated libs/common internal.ts to re-export from messaging instead of messaging-internal

The messaging library now exports both public APIs and internal implementations, which is a cleaner architecture than maintaining two separate libraries with circular dependencies.

Fixes rootDir configuration issues identified in the Nx library systematic fix project.

libs/messaging/src/helpers.spec.ts

@@ -0,0 +1,47 @@
+import { Subject, firstValueFrom } from "rxjs";
+
+import { getCommand, tagAsExternal } from "./helpers";
+import { isExternalMessage } from "./is-external-message";
+import { CommandDefinition, Message } from "./types";
+
+describe("helpers", () => {
+  describe("getCommand", () => {
+    it("can get the command from just a string", () => {
+      const command = getCommand("myCommand");
+
+      expect(command).toEqual("myCommand");
+    });
+
+    it("can get the command from a message definition", () => {
+      const commandDefinition = new CommandDefinition<Record<string, unknown>>("myCommand");
+
+      const command = getCommand(commandDefinition);
+
+      expect(command).toEqual("myCommand");
+    });
+  });
+
+  describe("tag integration", () => {
+    it("can tag and identify as tagged", async () => {
+      const messagesSubject = new Subject<Message<Record<string, unknown>>>();
+
+      const taggedMessages = messagesSubject.asObservable().pipe(tagAsExternal());
+
+      const firstValuePromise = firstValueFrom(taggedMessages);
+
+      messagesSubject.next({ command: "test" });
+
+      const result = await firstValuePromise;
+
+      expect(isExternalMessage(result)).toEqual(true);
+    });
+  });
+
+  describe("isExternalMessage", () => {
+    it.each([null, { command: "myCommand", test: "object" }, undefined] as Message<
+      Record<string, unknown>
+    >[])("returns false when value is %s", (value: Message<Record<string, unknown>>) => {
+      expect(isExternalMessage(value)).toBe(false);
+    });
+  });
+});