EEA: Only lock if entity instance exists in a transaction

[JAORMX]

Summary

This changes the logic to check that an entity exists as part of a transaction
before trying to lock it in the EEA. This way we'd avoid race conditions of trying
to lock an entity that gets deleted at the same time.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.669% (+0.02%) from 53.647%
when pulling d595513 on eea-entity-instance-get
into 1f9c1ef on main.