feat(ai): integrate Perplexity adapter

[Photon101]

Summary

• replace the Perplexity stub with a working Sonar API implementation
• update the deprecated llama-3.1-sonar-* default to the current Sonar model family
• map OpenAI-compatible response text and usage tokens, keep dry-run behavior, and pass Perplexity-specific options through opts.extra
• cover request shape, dry-run short-circuiting, and HTTP error excerpts with focused Vitest tests

This implements another AI provider adapter for #6.

Notes

Perplexity's current docs describe Sonar as OpenAI Chat Completions compatible, with https://api.perplexity.ai as the base URL and canonical POST /v1/sonar for direct API calls. The old llama-3.1-sonar-* API model names were deprecated, so this adapter now defaults to sonar-pro and includes sonar, sonar-pro, sonar-reasoning-pro, and sonar-deep-research.

Validation

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

As with #76, 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.