fix(ui): harden token costs refresh script

[Ghraven]

Problem

The token-cost refresh script fetches LiteLLM's model pricing JSON and writes model_costs.json, but the network request can hang indefinitely and HTTP failures are not surfaced explicitly. The generated JSON write also relied on the platform default text encoding.

Before / after

Before:

• requests.get() used no timeout.
• HTTP error responses were passed straight to JSON parsing.
• model_costs.json was written with the default platform encoding.

After:

• the request has a bounded timeout,
• HTTP failures raise through raise_for_status(), and
• the generated JSON is written as UTF-8.

Verification

• python -m py_compile telemetry/ui/scripts/token_costs.py
• Ran telemetry/ui/scripts/token_costs.py from a temporary working directory and confirmed it generated model_costs.json
• git diff --check