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.

.github/CODEOWNERS

@@ -96,7 +96,6 @@ libs/storage-core @bitwarden/team-platform-dev
 libs/logging @bitwarden/team-platform-dev
 libs/storage-test-utils @bitwarden/team-platform-dev
 libs/messaging @bitwarden/team-platform-dev
-libs/messaging-internal @bitwarden/team-platform-dev
 libs/serialization @bitwarden/team-platform-dev
 libs/guid @bitwarden/team-platform-dev
 libs/client-type @bitwarden/team-platform-dev