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.ts

@@ -0,0 +1,17 @@
+import { Subject } from "rxjs";
+
+import { getCommand } from "./helpers";
+import { MessageSender } from "./message.sender";
+import { CommandDefinition, Message } from "./types";
+
+export class SubjectMessageSender implements MessageSender {
+  constructor(private readonly messagesSubject: Subject<Message<Record<string, unknown>>>) {}
+
+  send<T extends Record<string, unknown>>(
+    commandDefinition: string | CommandDefinition<T>,
+    payload: Record<string, unknown> | T = {},
+  ): void {
+    const command = getCommand(commandDefinition);
+    this.messagesSubject.next(Object.assign(payload ?? {}, { command: command }));
+  }
+}