chat : honor json_schema in Ling 3.0 parser (#29813)
* chat : honor json_schema in Ling 3.0 parser
Ling 3.0 only built a grammar for tool calls and did not handle inputs.json_schema, so response_format requests were left unconstrained.
Add an eager response-format grammar path with precedence over tools, following the existing parser patterns. Require </think> before JSON when thinking is enabled and do not allow trailing prose after the JSON response.
Fixes #29652.
Assisted-by: Claude Opus 5.5
* chat : require Ling 3.0 think block for response formats
