chore(pricing): Gemini 3.7/3.6 Flash intro pricing doubles on 2027-01-01

[diillson]

googlePricing in cli/cost_tracker.go charges gemini-3.7-flash and gemini-3.6-flash at the introductory list price ($0.75/$3.75 per MTok), valid through 2026-12-31 per ai.google.dev pricing. From 2027-01-01 the list price doubles to $1.50/$7.50 (cache $0.15 + $1.00/1M/h).

On the flip date:

• Update the gemini-3.7-flash/gemini-3.6-flash case in googlePricing and the pins in cli/cost_tracker_pricing_test.go
• Re-check whether Google extended the promo or moved the models to another tier before applying