feat(ai): integrate DeepInfra adapter

[Photon101]

Summary

• replace the DeepInfra stub with a working OpenAI-compatible chat completions implementation
• update the default model to DeepInfra's documented deepseek-ai/DeepSeek-V3 sample model and include a small current model list
• preserve dry-run behavior, map usage tokens, pass provider-specific options through opts.extra, and surface concise HTTP error excerpts
• cover request shape, dry-run short-circuiting, and error handling with focused Vitest tests

This implements another AI provider adapter for #6.

Notes

DeepInfra documents an OpenAI-compatible base URL at https://api.deepinfra.com/v1/openai; this adapter posts to /chat/completions under that base and keeps baseUrl override support.

Validation

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

As with the other adapter PRs, the repo's full workspace install is currently blocked by the unrelated packages/dns/azure dependency on missing workspace package @sh1pt/core; I validated this package with root dev tooling and a local link to packages/core.