Play Framework

fix: findById(null) returns null; 500.json/xml/txt hide exception detail outside DEV (PF-198)

JPQL.findById returns null for a null id, as JPAModelLoader.findById does,
instead of Hibernate's "Identifier may not be null". A wrong-type id still
throws.

The default 500.json, 500.xml and 500.txt now show the exception only in DEV,
as 500.html does. Other modes get a fixed message and the incident id. The DEV
JSON is escaped with Gson so a multi-line message stays valid.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>