Add Perplexity integration attribution

[qirh]

Summary

• Add X-Pplx-Integration: ai-proxy to requests sent through the built-in Perplexity route.
• Preserve a caller-provided integration header.
• Leave every other provider route unchanged.

Verification

• Verified the Perplexity route adds the default header.
• Verified a lowercase caller override is preserved.
• Verified the OpenAI route receives no Perplexity attribution header.

[qirh]

@egoist small one for you whenever you have a moment: this adds an attribution header to the Perplexity route only, nothing else in the proxy changes. Could you confirm whether you are open to it, and approve the review workflow if so?