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/subject-message.sender.spec.ts

@@ -0,0 +1,58 @@
+import { bufferCount, firstValueFrom, Subject } from "rxjs";
+
+import { SubjectMessageSender } from "./subject-message.sender";
+import { CommandDefinition, Message } from "./types";
+
+describe("SubjectMessageSender", () => {
+  const subject = new Subject<Message<{ test: number }>>();
+  const subjectObservable = subject.asObservable();
+
+  const sut = new SubjectMessageSender(subject);
+
+  describe("send", () => {
+    it("will send message with command from message definition", async () => {
+      const commandDefinition = new CommandDefinition<{ test: number }>("myCommand");
+
+      const emissionsPromise = firstValueFrom(subjectObservable.pipe(bufferCount(1)));
+
+      sut.send(commandDefinition, { test: 1 });
+
+      const emissions = await emissionsPromise;
+
+      expect(emissions[0]).toEqual({ command: "myCommand", test: 1 });
+    });
+
+    it("will send message with command from normal string", async () => {
+      const emissionsPromise = firstValueFrom(subjectObservable.pipe(bufferCount(1)));
+
+      sut.send("myCommand", { test: 1 });
+
+      const emissions = await emissionsPromise;
+
+      expect(emissions[0]).toEqual({ command: "myCommand", test: 1 });
+    });
+
+    it("will send message with object even if payload not given", async () => {
+      const emissionsPromise = firstValueFrom(subjectObservable.pipe(bufferCount(1)));
+
+      sut.send("myCommand");
+
+      const emissions = await emissionsPromise;
+
+      expect(emissions[0]).toEqual({ command: "myCommand" });
+    });
+
+    it.each([null, undefined])(
+      "will send message with object even if payload is null-ish (%s)",
+      async (payloadValue) => {
+        const emissionsPromise = firstValueFrom(subjectObservable.pipe(bufferCount(1)));
+
+        sut.send("myCommand", payloadValue);
+
+        const emissions = await emissionsPromise;
+
+        expect(emissions[0]).toEqual({ command: "myCommand" });
+      },
+    );
+  });
+});