fix: merge additionalModelRequestFields instead of overwriting

When both reasoning_effort and extra_body are provided,
additionalModelRequestFields set by reasoning_effort (containing
reasoning_config) was silently overwritten by extra_body processing.
This prevented features like anthropic_beta for 1M context from
coexisting with reasoning_effort.

src/api/models/bedrock.py

@@ -874,8 +874,10 @@ class BedrockModel(BaseChatModel):
             }
 
             if additional_fields:
-                # Only set additionalModelRequestFields if there are actual fields to pass
-                args["additionalModelRequestFields"] = additional_fields
+                # Merge with existing additionalModelRequestFields (e.g., from reasoning_effort)
+                existing = args.get("additionalModelRequestFields", {})
+                existing.update(additional_fields)
+                args["additionalModelRequestFields"] = existing
 
                 # Extended thinking doesn't support both temperature and topP
                 # Remove topP to avoid validation error