Implement Reka chat completions adapter

[Photon101]

Summary

• replace the Reka AI stub with the documented /v1/chat/completions integration
• update the default model from stale reka-core to publicly documented reka-flash, with reka-edge aliases listed
• send system/user messages, max token and temperature options, plus provider-specific extra fields
• normalize assistant text and Reka input/output token usage
• expand tests from smoke-only to dry-run, request-shape, configured-base, usage mapping, and error coverage

Validation

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

Reference

• Reka chat completions docs: https://docs.reka.ai/chat/api-reference/create
• Reka model docs: https://docs.reka.ai/chat/models