Add response_format support to model settings

[juliendenize]

Why

Some models constrain their output to a JSON schema. To support this end-to-end, the encoded ModelSettings (which is what the tokenizer emits to condition the model) needs to carry the JSON schema derived from a request's response_format. This adds first-class response_format / JSON-schema support so those models can be driven correctly through mistral-common.

Summary

• Adds json_schema support to ResponseFormat:
  • New ResponseFormats.json_schema enum value.
  • New JsonSchema model (name, description, schema aliased to custom_schema, strict) with Draft 7 validation.
  • ResponseFormat.get_schema() resolves the effective schema: custom schema for json_schema, generic object/array anyOf for json_object, None for text.
• Adds json_schema field to encoded ModelSettings (tokenizer >= v15).
• Reworks ModelSettingsBuilder / FieldBuilder to a typed InputT -> OutputT convert-then-validate design so a request field (e.g. response_format) can be converted into a different settings field (e.g. json_schema), via _SETTINGS_TO_CONV_FIELDS_MAP.
• New JSONSchemaBuilder converts a ResponseFormat to a validated schema dict.
• New mistral_common/utils/json_utils.py with validate_json_schema_by_draft7.
• ChatCompletionRequest.to_openai now serializes by_alias=True so schema is emitted correctly.

Reviewer guide

Real logic:

• src/mistral_common/protocol/instruct/request.py — new JsonSchema, ResponseFormat.get_schema(), ModelSettings.json_schema, by_alias serialization.
• src/mistral_common/tokens/tokenizers/model_settings_builder.py — the FieldBuilder generic refactor (convert vs validate split) and new JSONSchemaBuilder + field mapping. Most careful review here.

New:

• src/mistral_common/utils/json_utils.py (+ utils/__init__.py) — Draft 7 schema validation helper.

Tests:

• tests/test_model_settings.py, tests/test_request.py, tests/test_json_utils.py, tests/test_converters.py, tests/test_tokenizer_v15.py — coverage for the new behavior.

Test plan

• pytest tests/test_model_settings.py tests/test_request.py tests/test_json_utils.py tests/test_converters.py tests/test_tokenizer_v15.py — 171 passed.
• ruff format --check, ruff check, mypy on changed sources — all clean.