fix: set additionalProperties: false for pydantic models with extra='ignore'

[octo-patch]

Fixes #1050
Related: #1125

Problem

When using a pydantic BaseModel (with the default extra='ignore' or explicit extra='ignore') as the schema for guidance.json(), the generated JSON grammar allows the LLM to produce arbitrary additional fields that pydantic would silently discard during validation.

This behaviour wastes tokens and surprises users who expect only the declared fields to be generated, as reported in #1050 and #1125.

Solution

Override model_schema() in GenerateJsonSchemaSafe to add "additionalProperties": false whenever the model's extra_fields_behavior is:

• None — the implicit pydantic default, which behaves as extra='ignore'
• "ignore" — explicitly set

Models with extra='allow' retain flexible schemas (pydantic sets additionalProperties: true).
Models with extra='forbid' already receive additionalProperties: false from pydantic itself.

The fix applies recursively to nested pydantic models within $defs, since each model's model_schema() is called independently.

Before / After

from pydantic import BaseModel
from guidance import json as gen_json

class MyModel(BaseModel):
    name: str
    age: int

grammar = gen_json(schema=MyModel)

# Before this fix:
grammar.match('{"name": "Alice", "age": 30, "extra": true}')  # -> Match (allowed!)

# After this fix:
grammar.match('{"name": "Alice", "age": 30, "extra": true}')  # -> None (rejected)
grammar.match('{"name": "Alice", "age": 30}')                 # -> Match (correct)

Testing

Added TestAdditionalProperties to tests/unit/library/test_pydantic.py covering:

• Default extra rejects additional properties
• Explicit extra='ignore' rejects additional properties
• extra='allow' still permits additional properties
• Nested models also reject additional properties