add warning message to ensure rosidl_adaptor is available.

[fujitatomoya]

Description

Fixes #885

Is this user-facing behavior change?

No

Did you use Generative AI?

No

Additional Information

[Copilot]

Pull Request Overview

This PR adds a warning message to inform users when rosidl_adapter is not available but non-IDL interface files are provided, helping them understand why their build might fail and how to resolve the issue.

• Adds a warning message in the rosidl_generate_interfaces macro when rosidl_adapter is not found
• Provides clear guidance on how to resolve the issue by adding the required package dependency

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[fujitatomoya]

Pulls: #886
Gist: https://gist.githubusercontent.com/fujitatomoya/fdd51a657e3e7a5b7cbee9428faec2bf/raw/afbf7df8e61ab7ae002ecd54d0162b005a8c730f/ros2.repos
BUILD args: --packages-above-and-dependencies rosidl_cmake
TEST args: --packages-above rosidl_cmake
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/16784

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[GuillaumeDua]

LGTM 👍 , but perhaps there's other generators and/or a specific generator API worth mentioning here ?