Improve diagnostic messages in SBGN AF Schematron validation rules

[Alphabeas]

Summary

This PR improves the diagnostic error messages in the SBGN Activity Flow
Schematron validation rules.

Some of the existing validation assertions contained very short or generic
messages, which can make it difficult for users to understand why validation
fails when creating SBGN diagrams.

Changes

This update improves several assertion messages by:

• Providing clearer explanations of validation failures
• Using more descriptive domain terminology
• Making validation feedback easier to understand for users

Motivation

Clear diagnostic messages are important for helping users quickly identify
and fix issues in SBGN diagrams. Improving these messages enhances the
usability of the SBGN validation system and benefits tools such as Newt
and other SBGN editors that rely on these validation rules.

Scope

This PR only modifies the diagnostic text of existing assertions.
No validation logic or rule conditions were changed.

[Alphabeas]

@cannin Hi, this is my first contribution to the project. Could you please approve the workflows so that the CI checks can run? Thanks!