MAINT: Remove deprecated message init in JSON schema normalizer

[biefan]

Description

• Update JsonSchemaNormalizer to construct Message with keyword-only arguments.
• Update the JsonSchemaNormalizer unit tests to use the same non-deprecated Message construction style.
• This removes the deprecation warnings for this normalizer ahead of the 0.16.0 removal path.

Tests and Documentation

• uv run -m pytest tests/unit/message_normalizer/test_json_schema_normalizer.py -W default
• uv run -m pytest tests/unit/message_normalizer -W default
• uv run pre-commit run --files pyrit/message_normalizer/json_schema_normalizer.py tests/unit/message_normalizer/test_json_schema_normalizer.py
• uv run -m ty check pyrit/message_normalizer/json_schema_normalizer.py tests/unit/message_normalizer/test_json_schema_normalizer.py
• uv run -m pytest tests/unit/message_normalizer/test_json_schema_normalizer.py -W error::DeprecationWarning

Documentation changes are not applicable for this maintenance-only update.