Build failure due to missing test_interface_files when following README manual setup

[attu0]

Title: Build failure due to missing test_interface_files when following README setup

Description:

While following the manual setup instructions in the README (without using the .repos file), the build fails due to a missing dependency.

Error:
Could not find a package configuration file provided by "test_interface_files"

Steps to reproduce:

1. Follow README manual cloning steps
2. Run colcon build

Observed behavior:

• Build fails at test_msgs due to missing test_interface_files

Expected behavior:

• Setup instructions should include all required dependencies OR clearly recommend using the .repos file

Environment:

• OS: Ubuntu 22.04
• ROS 2: Humble
• Build system: colcon

Additional context:
This issue does not occur when using:
vcs import src < ros2_rust_humble.repos

Possible causes:

• Missing dependency in manual setup instructions
• Inconsistency between README and .repos workflow

Related PR: #615