fix: preserve Perplexity usage cost in non-strict mode

[voodoohop]

• Preserves full Perplexity usage in non-strict non-stream responses so downstream billing can read provider-reported request cost.
• Keeps strict OpenAI compliance token-only for non-stream and stream responses.
• Adds focused transform tests for non-strict passthrough and strict stripping.

Needed by pollinations/pollinations search billing work so non-stream Perplexity requests can bill from usage.cost.request_cost.