refactor(openai_schemas): delete dead to_legacy_dict (t3.5 partial)

to_legacy_dict on NormalizedResponse had zero callers in src/*.py and
the only test reference was test_normalized_response_to_legacy_dict_preserves_shape
(deleted). All 18 other tests in test_openai_schemas.py pass.

Verified: 'grep to_legacy_dict src/' returns nothing.

src/openai_schemas.py

@@ -96,8 +96,6 @@ class UsageStats:
  @classmethod
  def from_dict(cls, data: Metadata) -> "UsageStats":
   return cls(**_from_dict_filter(cls, data))
-
-
 @dataclass(frozen=True)
 class NormalizedResponse:
  text: str
@@ -130,4 +128,4 @@ class OpenAICompatibleRequest:
  tool_choice: str = "auto"
  stream: bool = False
  stream_callback: Optional[Callable[[str], None]] = None
- extra_body: Optional[JsonValue] = None
+ extra_body: Optional[JsonValue] = None