feat(providers): add Perplexity integration attribution header

[qirh]

Summary

• Perplexity would like to identify requests made through OmniRoute's Perplexity integration.
• Add X-Pplx-Integration: omniroute to the perplexity (Sonar chat completions) and perplexity-agent (Responses) registry entries via the existing headers field, and to the Perplexity Search request builder in open-sse/handlers/search/perplexitySearch.ts.
• Leave every other provider and request behavior unchanged. The perplexity-web session provider is not touched.

Related Issues

• None.

Validation

• Change type: provider
• Focused tests: node --import tsx/esm --test tests/unit/perplexity-agent-provider.test.ts tests/unit/search-handler-perplexity-options.test.ts tests/unit/provider-translate-path-golden.test.ts
• npm run lint (eslint on the touched files) and prettier --check
• Reconciled with the current active release base (release/v3.8.52)
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/perplexity-agent-provider.test.ts: asserts DefaultExecutor headers for perplexity and perplexity-agent include X-Pplx-Integration: omniroute.
• tests/unit/search-handler-perplexity-options.test.ts: asserts the Perplexity Search request carries the header.
• tests/snapshots/provider/translate-path.json: regenerated with UPDATE_GOLDEN=1; only the perplexity and perplexity-agent entries change.

Coverage Notes

• The registry headers change is covered by the executor header test and the translate-path golden snapshot; the search change is covered by the search handler test.

Reviewer Notes

• Header only. No routing, auth, or payload changes.