feat: support Responses to Chat (#5787)

* fix(openai): harden Chat-to-Responses compatibility

Add a shared Responses-to-Chat stream state machine and use it from the OpenAI relay path. Preserve assistant text alongside tool calls, bind tool argument deltas by output_index, map incomplete finish reasons, support reasoning/custom tool events, and buffer upstream SSE for non-stream Chat clients.

Add deterministic service tests and relay SSE tests for the conversion path.

Related to #5745.

* refactor: rename openaicompat to relayconvert for improved clarity

* feat(gemini): support responses request conversion

* feat: add responses to chat conversion support

* fix: harden responses chat conversion edge cases

service/convert.go

@@ -628,12 +628,14 @@ func ResponseOpenAI2Claude(openAIResponse *dto.OpenAITextResponse, info *relayco
 			claudeContent.Type = "tool_use"
 			claudeContent.Id = toolUse.ID
 			claudeContent.Name = toolUse.Function.Name
-			var mapParams map[string]interface{}
-			if err := common.Unmarshal([]byte(toolUse.Function.Arguments), &mapParams); err == nil {
-				claudeContent.Input = mapParams
-			} else {
-				claudeContent.Input = toolUse.Function.Arguments
+			mapParams := map[string]interface{}{}
+			if strings.TrimSpace(toolUse.Function.Arguments) != "" {
+				var parsed map[string]interface{}
+				if err := common.Unmarshal([]byte(toolUse.Function.Arguments), &parsed); err == nil && parsed != nil {
+					mapParams = parsed
+				}
 			}
+			claudeContent.Input = mapParams
 			contents = append(contents, claudeContent)
 		}
 	}