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.ts

@@ -0,0 +1,20 @@
+import { map } from "rxjs";
+
+import { EXTERNAL_SOURCE_TAG } from "./is-external-message";
+import { CommandDefinition } from "./types";
+
+export const getCommand = (
+  commandDefinition: CommandDefinition<Record<string, unknown>> | string,
+) => {
+  if (typeof commandDefinition === "string") {
+    return commandDefinition;
+  } else {
+    return commandDefinition.command;
+  }
+};
+
+export const tagAsExternal = <T extends Record<PropertyKey, unknown>>() => {
+  return map((message: T) => {
+    return Object.assign(message, { [EXTERNAL_SOURCE_TAG]: true });
+  });
+};