feat: add RainDragon provider with Gemma 4 26B and 31B mappings

[hemaprathaban]

Adds RainDragon as an inference provider, with mappings for Gemma 4 26B and Gemma 4 31B.

Public sources for the values in this PR:

• API base URL and sign-up: https://inference.raindragon.ai
• Pricing and context length: https://inference.raindragon.ai/v1/models (public, no login needed). Gemma 4 26B $0.075 in / $0.35 out per M tokens, Gemma 4 31B $0.13 / $0.38, cached input $0.03/M.
• Privacy policy: https://inference.raindragon.ai/privacy
• Region: inference runs in the US (us-east).
• Rate limits: not published, so set to null as the schema allows.

Happy to change anything you need.