Hardening: validateJsonSchema() must check containment

Problem/Motivation

Codex identified the following issue:

     The defaults applicator protects $ref file loading with realpath() containment in dev/json_schema_validator/src/
     SchemaDefaultsApplicator.php:560, with a traversal test in dev/json_schema_validator/tests/src/Kernel/
     JsonSchemaValidatorDefaultsTest.php:212. Plain validateJsonSchema() delegates file resolution to Opis, whose resolver maps URL
     paths to files and reads them directly. Current call sites mostly pass enum-controlled schema names, so exposure is low, but
     validate/reject schema names containing .., %, ://, or leading slashes, or use the same contained loader path for plain validation.