Fix A2A streaming chunk parsing and context_id persistence in OpenStudio MCP Demo UI

[Copilot]

The OpenStudio MCP demo UI was only checking chunk.get("content") on streaming responses, missing the actual A2A JSON-RPC envelope structure and never persisting context_id, causing empty responses and broken multi-turn conversations.

Changes

• Chunk parsing — replaced naive content field access with full A2A JSON-RPC result parsing, branching on result.kind:
  • artifact-update → extracts text from artifact.parts
  • status-update → extracts text from status.message.parts
• context_id persistence — captures result.contextId on each chunk and stores it in st.session_state["context_id"] for follow-up turns
• Fallbacks — retains handling for delta.text, message.text, content, and data chunk shapes for robustness

if isinstance(chunk, dict) and "result" in chunk:
    result = chunk.get("result", {})
    context_id = result.get("contextId")
    if context_id:
        st.session_state["context_id"] = context_id

    kind = result.get("kind")
    if kind == "artifact-update":
        parts = result.get("artifact", {}).get("parts", [])
        ...
    elif kind == "status-update":
        parts = result.get("status", {}).get("message", {}).get("parts", [])
        ...

Mirrors the pattern already established in examples/eplus_mcp_demo/ui.py.

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