Mirrors the CLI fix for the gateway /model handler. When the user is on
a custom provider (provider=custom, localhost, or 127.0.0.1 endpoint),
/model <name> no longer tries to auto-detect a provider switch.
Previously, typing /model openrouter/nvidia/nemotron:free on Telegram
while on a localhost endpoint would silently accept the model name on
the local server — auto-detection failed to match the free model, so
the provider stayed as custom with the localhost base_url. The user saw
'Model changed' but requests still went to localhost, which doesn't
serve that model.
Now shows the endpoint URL and provider:model syntax tip, matching
the CLI behavior.
f3b2303428