Add Transaction.setGeneratedPropertiesEnabled(boolean)

[rbygrave]

Adds transaction-level control over whether Ebean auto-generates values for @WhenCreated, @WhenModified, @WhoCreated and @WhoModified properties.

Motivation

Backup/restore scenarios need to preserve the original audit timestamps and user values when re-inserting exported data. Without this, every save overwrites those fields with the current time/user.

Usage

 try (Transaction txn = DB.beginTransaction()) {
   txn.setGeneratedPropertiesEnabled(false);
   bean.setWhenCreated(originalTimestamp);
   bean.setWhenModified(originalTimestamp);
   DB.save(bean);
   txn.commit();
 }

Behaviour

• Disabled (false): generated property values are only written if the property currently has a null value. Any non-null value set on the bean is preserved.
• @Version is unaffected: the version property always auto-increments regardless of this setting, preserving optimistic locking integrity.
• Default is true: all existing behaviour is unchanged.

Files changed

• Transaction — new setGeneratedPropertiesEnabled(boolean) with javadoc
• SpiTransaction — new isGeneratedPropertiesEnabled()
• SpiTransactionProxy — delegates both methods
• JdbcTransaction — field + implementation (default true)
• NoTransaction, ImplicitReadOnlyTransaction — no-op setter, true getter
• PersistRequestBean — onInsertGeneratedProperties, onUpdateGeneratedProperties, onFailedUpdateUndoGeneratedProperties all gate on isGeneratedPropertiesEnabled()
• TestGeneratedProperties — 3 new tests covering insert-preserves, insert-null-still-filled, and update-preserves

Supersedes

PR #2943 — same feature, renamed from setOverwriteGeneratedProperties to setGeneratedPropertiesEnabled for a clearer, positive-sense API.

[rbygrave]

This PR replaces #2943