Python: DevUI Fix Serialization, Timestamp and Other Issues (#1584)

* refactor(devui): adopt standard OpenAI lifecycle events for agents and workflows

- Replace custom workflow events with OpenAI Responses API standard lifecycle events
- Add AgentStartedEvent, AgentCompletedEvent, AgentFailedEvent for clean separation
- Implement ExecutorActionItem for workflow executor tracking
- Convert informational events to trace events to reduce noise
- Update README mapper table with comprehensive event mappings
- Maintain full backward compatibility with legacy events

* fix(devui): resolve timestamp overwriting and Content serialization errors

- Fix tool call timestamps being overwritten on each render (#1483)
- Add recursive Content serialization to handle ChatMessage and nested objects (#1548)
- Implement proper MCP tool cleanup on server shutdown
- Add timestamp field to function_result.complete events
- Enhance credential and client resource cleanup

Fixes #1483, #1548
Partial improvements for #1476

python/packages/devui/frontend/src/services/api.ts

@@ -397,9 +397,7 @@ class ApiClient {
     // Convert to OpenAI format - use model field for entity_id (same as agents)
     const openAIRequest: AgentFrameworkRequest = {
       model: workflowId, // Use workflow ID in model field (matches agent pattern)
-      input: typeof request.input_data === 'string'
-        ? request.input_data
-        : JSON.stringify(request.input_data || ""), // Convert input_data to string
+      input: request.input_data || "", // Send dict directly, no stringification needed
       stream: true,
       conversation: request.conversation_id, // Include conversation if present
     };