fix(openai): resolve Anthropic compat mapping from normalized model

Anthropic compat requests normalize reasoning suffixes before forwarding, but the account mapping step was still using the raw request model. Resolve billing and upstream models from the normalized compat model so explicit account mappings win over fallback defaults.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent

)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/service/openai_gateway_messages.go

@@ -41,6 +41,7 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 	}
 	originalModel := anthropicReq.Model
 	applyOpenAICompatModelNormalization(&anthropicReq)
+	normalizedModel := anthropicReq.Model
 	clientStream := anthropicReq.Stream // client's original stream preference
 
 	// 2. Convert Anthropic → Responses
@@ -60,13 +61,14 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 	}
 
 	// 3. Model mapping
-	billingModel := resolveOpenAIForwardModel(account, originalModel, defaultMappedModel)
+	billingModel := resolveOpenAIForwardModel(account, normalizedModel, defaultMappedModel)
 	upstreamModel := resolveOpenAIUpstreamModel(billingModel)
 	responsesReq.Model = upstreamModel
 
 	logger.L().Debug("openai messages: model mapping applied",
 		zap.Int64("account_id", account.ID),
 		zap.String("original_model", originalModel),
+		zap.String("normalized_model", normalizedModel),
 		zap.String("billing_model", billingModel),
 		zap.String("upstream_model", upstreamModel),
 		zap.Bool("stream", isStream),