fix: accept LF-terminated Perplexity streams

[voodoohop]

• Fixes Perplexity streams being buffered as one JSON object when upstream uses LF instead of CRLF. Accepts both separators; keeps raw SSE valid when no transformer is selected.
• Keeps the existing Perplexity adapter, developer-to-system conversion, model routes, OpenRouter fallbacks, prices, usage validation, and retries unchanged. The compatible-adapter alternative rejected developer messages, so it was discarded.
• Reproduced the exact JSON parse failure with a captured live response. Five framing regression tests pass (LF/CRLF, byte-split UTF-8, raw passthrough); the LF tests fail on the old parser.
• Seven live requests through local patched Portkey and Gen’s existing client/validator pass: all three Sonar models, streaming/non-streaming, high-search citations and usage, plus a developer-message request. Non-stream cost metadata stripping remains separate in fix: preserve Perplexity usage cost in non-strict mode #7.
• Validation: 52 focused tests, formatting, diff check, and build pass. Full-suite setup failures remain; type checking reports the same 12 errors on unchanged main. No production deployment or secret changes. Complements feat: add OpenRouter fallbacks for Perplexity Sonar models pollinations#14642.