🐛 Declare main edflex table to fix course deletion

[DamienVauchel]

🎯 Purpose of this Pull Request

The plugin never declared its mandatory main edflex table. Moodle's core remove_course_contents() runs SELECT … FROM mdl_{modname} for every installed module on course deletion, so the missing table raised a dmlreadexception that broke deletion of any course platform-wide — even courses that never used Edflex.

Related to #5

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📝 Summary of Changes

• Declare the main edflex table in db/install.xml (id, course, name, intro, introformat, timemodified).
• Create the table on already-installed v1.0.0 instances in db/upgrade.php, guarded by table_exists(); bump $plugin->version to 2026062400.
• edflex_delete_instance() now purges its {edflex} row instead of returning false.

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✔️ Moodle Plugin Checklist

Code Quality & Standards

• Code follows Moodle coding guidelines
• PHPDoc blocks updated where needed
• No debug statements left
• Namespace and file structure follow Moodle plugin conventions

Functional Changes

• Upgrade steps added to db/upgrade.php

Database & Files

• DB schema changes added to db/install.xml and db/upgrade.php

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🧪 Testing

Manual Tests

• On an instance upgraded from v1.0.0: run the plugin upgrade, confirm the edflex table is created.
• Create any course (no Edflex activity needed) and delete it → succeeds, no dmlreadexception.
• Fresh install: confirm the edflex table is present after install.

Automated Tests

• No unit test added — change is schema/upgrade only; moodle-plugin-ci (validate + savepoints) covers it on CI.

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🔄 Regression Risk

Additive schema only. Touches the upgrade path for existing installs and the course-deletion path (the fix target). Low risk.

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🔗 Additional Notes

• Moodle plugin CI is expected to catch a missing main table — worth understanding why it passed before.
• Follow-up, out of scope: add backup/restore + privacy provider once the edflex table actually stores instance data (today edflex_add_instance still returns false, so no data is written yet).