[Saleh Alghusson (qirh)]

Adds an X-Pplx-Integration: agent365-samples default header to the Node.js Perplexity sample's SDK client so Perplexity can recognize API traffic coming from this sample.

• The header is only attached when the client's resolved base URL host is exactly api.perplexity.ai. If PERPLEXITY_BASE_URL points to a proxy or other host, the client is unchanged.
• No change to authentication, model selection, or request payloads.

Verification:

• npm install and npx tsc -p . in nodejs/perplexity/sample-agent succeed.
• Local harness with a stubbed fetch against the built client: header sent once for the default and explicit https://api.perplexity.ai base URLs; not sent for a custom proxy host or a lookalike host (api.perplexity.ai.evil.com); Authorization unchanged in all cases (12/12 checks).
• Prettier reports no issues on the added lines.

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