[ty] Pydantic: Warn about immediately discarded extra arguments (#26762)
## Summary
By default, Pydantic allows arbitrary extra constructor arguments, which
is simply ignored (`extra="ignore"`). This often indicates a mistake
though, so we now emit emit a new diagnostic with severity "warning":
```py
class Person(BaseModel):
name: str
admin: bool = Field(default=false)
# a typo: should be `admin`, not `admni`
Person(name="Alice", admni=true) # warning: [pydantic-discarded-extra-argument]
```
closes https://github.com/astral-sh/ty/issues/3966
## Ecosystem impact
One true positive.
## Test plan
New and updated Markdown tests