[SYNCOPE-1980] Fix empty audit history diff under Jackson 3

[ozimakov]

SYNCOPE-1980

Problem

In the admin console, the per-entity "Audit History" modal (User / Group / Any Object / Realm / Policy → View Audit History) never renders a JSON diff: both the "previous" and "after" panes are always empty, for any selected versions.

Root cause

AuditHistoryDetails#toJSON deserializes the audited entity with an untyped reader:

T entity = MAPPER.reader().
        with(StreamReadFeature.STRICT_DUPLICATE_DETECTION).
        readValue(content);

Under Jackson 3 an ObjectReader with no configured value type throws InvalidDefinitionException: No value type configured for ObjectReader. The exception is swallowed by the surrounding try/catch and toJSON returns an empty Model, so the failure is silent in the UI and only visible in the console log. This is a regression from the Jackson 2 → 3 migration (tools.jackson.*): Jackson 2's mapper.reader() tolerated a missing root type, Jackson 3 requires one.

Fix

Read into the concrete type of the entity, which the panel already holds as currentEntity:

@SuppressWarnings("unchecked")
T entity = (T) MAPPER.readerFor(currentEntity.getClass()).
        with(StreamReadFeature.STRICT_DUPLICATE_DETECTION).
        readValue(content);

The JSON's _class is then consumed as an ordinary property and the entity deserializes correctly for User/Group/AnyObject/Realm/Policy.

Testing

Verified locally against the standalone (H2) distribution: the diff renders and the InvalidDefinitionException no longer appears in the console log. Full reactor test suite passes.

[ilgrosso]

@ozimakov thanks for this fix.

One small note: since Syncope 4.0, the local distribution is not based on H2 but embedded PostgreSQL.