[FEATURE]: Add Snowflake Cortex provider

[kameshsampath]

Snowflake Cortex exposes an OpenAI-compatible Chat Completions API but requires a few code changes to work with opencode:

1. The endpoint URL is per-account (https://${SNOWFLAKE_ACCOUNT}.snowflakecomputing.com/api/v2/cortex/v1) — it can't be a static models.dev URL
2. The API uses max_completion_tokens instead of max_tokens
3. It returns HTTP 400 "conversation complete" as a normal stop condition rather than a finish_reason

A companion models.dev PR adds the catalog entry with all models and capabilities: anomalyco/models.dev#1902

I have verified this feature hasn't been suggested before.

[kameshsampath]

CC: @sfc-gh-kkeller