15498: Enable Pydantic polymorphic_serialization flag on response serialization

[valentinDruzhinin]

Feature: Polymorphic Response Model Serialization

Adds support for response_model_polymorphic_serialization parameter to FastAPI path operations, enabling polymorphic serialization when returning subclass instances through base-class-typed response models.
Implements: #15498

What it does:

• Default behavior (False): When returning a subclass instance, only base class fields are serialized in the response
• With polymorphic flag (True): Subclass-specific fields are included in the serialized response

Example:

class User(BaseModel):
    name: str

class UserLogin(User):
    password: str

@app.get("/user", response_model=User, response_model_polymorphic_serialization=True)
def get_user():
    return UserLogin(name="pydantic", password="secret")
    # Response includes: {"name": "pydantic", "password": "secret"}

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing valentinDruzhinin:feature/15498-pylymorphic_serialization (e41b2af) with master (5d5666b)¹}

Footnotes

1. No successful run was found on master (e89a37e) during the generation of this report, so 5d5666b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩