feat(core): update Copilot for token-based billing

[rekram1-node]

• move Copilot requests to the new API version and load available models/pricing from /models
• handle incomplete model records without dropping dynamic discovery,
• use Copilot utility models for session titles without exposing hidden models in the picker
• track billed session cost from copilot_usage.total_nano_aiu, with advertised model prices kept as a fallback
• avoid treating Copilot cached-input pricing as a cache-write rate when the API does not return one
• switch to UBB

Remaining work:

• long context pricing opt in (need to figure out how to display that to users)

closes: #30092