feat: add stream field to UserInput for body-controlled SSE streaming

[hillhack]

Summary of Changes

File	Change
response_model.py	Added stream: bool = False to UserInput.
conversations.py	Introduced a unified POST /conversations/agent-retriever endpoint. Returns StreamingResponse when stream=true and ChatResponse when stream=false. Set response_model=None to skip FastAPI schema inference for union responses. Kept /agent-retriever/stream as a backward-compatible alias.
ui.py	Updated proxy logic to inspect stream in the request body. Uses httpx streaming context manager (timeout=None) for SSE passthrough when stream=true; falls back to standard buffered response otherwise.
test_api_conversations_streaming.py	Updated all existing UserInput(...) calls to explicitly set stream=False. Added two tests in TestUnifiedEndpointStreamBranching: one verifies stream=true returns StreamingResponse, the other verifies stream=false returns ChatResponse.

---

Usage

Non-Streaming (default behavior)

{ "query": "How do I install OpenROAD?", "stream": false }```

###Streaming (SSE)

```POST /conversations/agent-retriever
{ "query": "How do I install OpenROAD?", "stream": true }```
https://github.com/The-OpenROAD-Project/ORAssistant/issues/157