Add Configurable User Prompt Field to Agentic Subprocess Properties Panel

[pieter-schutte]

Description of Problem

Based on finos/fluxnova-plugins#53, back-end implementation finos/fluxnova-plugins#59

Description

As a process modeler, I want to configure a user prompt for agentic subprocesses in the BPMN model, so that I can specify the initial user message the agent receives for my specific process.

Currently, the subprocess properties panel allows configuration of systemPrompt (which defines AI behavior). We need to add a complementary userPrompt field that allows modelers to specify the initial user message sent to the agent. The field should support ${...} expressions for instance-specific prompt customization, with fallback to the default when empty.

Potential Solutions

Acceptance Criteria

• A new text input field labeled "User Prompt" appears in the subprocess properties panel for agentic subprocesses
• The field is positioned logically near the existing "System Prompt" field
• The field accepts multi-line text input (supports line breaks via Enter)
• The field supports ${...} expressions for runtime variable substitution (e.g., Hello ${userName})
• The field validates against:
  • Null/empty prompt values (should be allowed; treated as empty string and will fallback to default at runtime)
  • Special characters (should be allowed; ensure proper escaping for XML serialization)
• Field input type matches the existing system prompt field (same component/styling)
• Changes to the field are persisted when the model is saved
• The modeler gracefully handles models that don't have a userPrompt configured (backwards compatibility)

Technical Notes

• The userPrompt value should be stored as a BPMN extension attribute in the agentic namespace (same approach as systemPrompt)
• The field should be created using the same underlying type as the system prompt field
• No default value should be applied at the UI level; if the field is left empty, it should serialize as an empty string
• Document to users that:
  • ${variableName} expressions can be used for dynamic prompt values
  • Expressions can be mixed with normal text (e.g., "Process: ${processName}, User: ${userId}")
  • Empty prompts will result in fallback to the default message at runtime

Definition of Done

• Code reviewed and approved by modeler team lead
• Unit/integration tests added for field persistence and validation
• Expression syntax is properly preserved in XML serialization
• Backwards compatibility verified with existing BPMN files