fix(structured outputs): send structured output beta header when format is omitted (#1158)

This reverts commit 062077e.

Author: karpetrosyan

src/anthropic/resources/beta/messages/messages.py

@@ -1118,7 +1118,7 @@ def parse(
 
         betas = [beta for beta in betas] if is_given(betas) else []
 
-        if "structured-outputs-2025-12-15" not in betas and is_given(output_format):
+        if "structured-outputs-2025-12-15" not in betas:
             # Ensure structured outputs beta is included for parse method
             betas.append("structured-outputs-2025-12-15")
 
@@ -2813,7 +2813,7 @@ async def parse(
             )
         betas = [beta for beta in betas] if is_given(betas) else []
 
-        if "structured-outputs-2025-12-15" not in betas and is_given(output_format):
+        if "structured-outputs-2025-12-15" not in betas:
             # Ensure structured outputs beta is included for parse method
             betas.append("structured-outputs-2025-12-15")