fix: harden token cost refresh script

[Ghraven]

Problem

telemetry/ui/scripts/token_costs.py refreshes pricing metadata from LiteLLM using an unbounded requests.get() call. If the remote endpoint stalls, the script can hang indefinitely. It also writes the generated JSON without an explicit text encoding, which makes the output depend on the platform default encoding.

Before / after

Before:

• remote fetches had no timeout
• HTTP error responses were passed to response.json()
• model_costs.json was written with the process default text encoding

After:

• the fetch is bounded with a 30 second timeout
• HTTP failures are surfaced via raise_for_status() before parsing JSON
• the generated JSON is written with encoding="utf-8"

Verification

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