Fix load_demo_single called with list instead of Context in gradio_web_server

[Chessing234]

Fixes #3788.

Bug

load_demo in fastchat/serve/gradio_web_server.py invokes
load_demo_single(models, url_params), passing the global models list as
the first positional argument. load_demo_single now expects a Context
object and dereferences context.text_models / context.models, so the
call raises AttributeError: 'list' object has no attribute 'text_models'
when Gradio fires the demo-load callback (including the --model-list-mode reload path).

Root cause

Commit 2c68a13 (Code sync, PR #3546) changed load_demo_single's
signature from (models, url_params) to (context: Context, query_params)
and moved the body to read context.text_models / context.models, but
load_demo in the same file was not updated and still passes the plain
list.

Why the fix is correct

In the standalone (non-multi) server there is no text/vision distinction,
so wrapping the existing models list as both text_models and models
on a fresh Context preserves prior behavior exactly: load_demo_single
selects the default model from context.text_models and populates the
dropdown choices from context.models, which both map to the same list
that was previously used directly. Context is already imported in this
file (line 37), and the other caller in gradio_web_server_multi.py
continues to pass a fully-populated Context unchanged.