Add minimal controller endpoints for LLM chatbot (HTTP test first)

Add minimal controller endpoints for LLM chatbot (HTTP test first)

As part of building the LLM chatbot, I’m adding minimal controller methods first to enable frontend ↔ backend HTTP tests.
This change only exposes simple endpoints (e.g., GET /experiment/agent/modelInfo, POST /experiment/agent/generate-content) with basic validation and mock responses—no service/DB/LLM integration yet.
We’ll iterate and harden the logic in follow-up tasks.

- Unblock frontend integration and smoke testing
- Defer business logic to later PRs

Issue-ID:AIMLFW-235
Change-Id: I5a142ab8cedc70b0cf6eb37fafd59b9250882046
Signed-off-by: alswp006 <alswp006@gmail.com>