Implement Cohere v2 chat adapter

[Photon101]

Summary

• replace the Cohere AI stub with a real POST /v2/chat integration
• update the default model from deprecated command-r-plus to current command-a-03-2025, with current Command model IDs listed
• send system/user messages, max token and temperature options, plus provider-specific extra fields
• normalize text content and token usage from Cohere v2 responses
• expand tests from smoke-only to dry-run, request-shape, usage mapping, billed-unit fallback, and error coverage

Validation

• corepack pnpm exec vitest run packages/ai/cohere/src/index.test.ts
• corepack pnpm exec tsc -p packages/ai/cohere/tsconfig.json --noEmit
• git diff --check

Reference

• Cohere v2 Chat docs: https://docs.cohere.com/v2/reference/chat
• Cohere model docs: https://docs.cohere.com/v2/docs/models