Introduce a new Perplexity provider that targets Perplexity's
OpenAI-compatible chat-completions endpoint at https://api.perplexity.ai.
Models exposed: sonar, sonar-pro, sonar-reasoning, sonar-reasoning-pro
(all 128k context).
API key resolution order:
1. perplexityApiKey from settings
2. PERPLEXITY_API_KEY env var
3. PPLX_API_KEY env var (fallback)
Wired into the provider registry (packages/types), api factory,
profile validation, webview settings UI, model picker, validation,
and English locale strings.
Tests cover construction, base URL/auth, env-var fallbacks, model
selection, streaming, and error propagation.
