Add support for recursive Pydantic models

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**Is your feature request related to a problem? Please describe.**
Yes. I'm encountering a problem when trying to use recursive Pydantic models as a response schema for `google.genai.Client`. Specifically, when using a recursive model (e.g., an `AND` or `OR` block that can contain nested instances of itself), the call to `generate_content` fails with a `RecursionError: maximum recursion depth exceeded`.

**Describe the solution you'd like**
I would like the SDK and/or backend to support recursive Pydantic models correctly in the `response_schema` argument for structured generation using Gemini models. This is critical for representing complex hierarchical or tree-structured data (e.g., nested guidelines with logical operators like AND/OR).

**Describe alternatives you've considered**
- Flattening the model structure manually, which becomes unmanageable and error-prone for deeply nested logical rules.
- Using a post-processing step to manually reconstruct the recursive data structure from a less structured JSON output, which defeats the purpose of using `response_schema`.

**Additional context**
Here is a simplified example of what I am trying to achieve:

```python
from __future__ import annotations

from google import genai
from google.genai.types import GenerateContentConfig
from pydantic import BaseModel

class Condition(BaseModel):
condition: str

class ANDBlock(BaseModel):
AND: list[ANDBlock | ORBlock | Condition]

class ORBlock(BaseModel):
OR: list[ANDBlock | ORBlock | Condition]

class Conditions(BaseModel):
conditions: ANDBlock | ORBlock

client = genai.Client(api_key='...')

response = client.models.generate_content(
model="gemini-2.5-flash",
contents="""Please structure the following rules into logical groups using AND/OR:
- The user must be over 18 years old.
- The user must have a valid driver's license or state-issued ID
- The user must not have committed any misdemeanor in last 18 months and the user must not have been fined in last 24 months.
- The user must have proof of insurance.
- A background check must be completed.
""",
config=GenerateContentConfig(
system_instruction="Convert the rules into a logical tree using nested AND/OR blocks.",
response_mime_type="application/json",
response_schema=Conditions
)
)
```

This raises following exception:

```RecursionError: maximum recursion depth exceeded```