Implement Venice chat completions adapter

[Photon101]

Summary

• replace the Venice AI stub with a real OpenAI-compatible chat completions adapter
• use the current Venice /api/v1/chat/completions endpoint, documented model IDs, dry-run behavior, standard generation options, and opts.extra passthrough for Venice-specific parameters
• add focused tests for dry-run behavior, request payload/auth, token usage mapping, and HTTP error excerpts

References

• https://docs.venice.ai/api-reference/endpoint/chat/completions
• https://docs.venice.ai/overview/about-venice

Validation

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