Add Reasoning Tokens to Perplexity Provider Metadata

[liby]

Feature Description

Currently, the Perplexity provider in AI SDK returns limited usage metadata. For comprehensive cost tracking and optimization, propose extending the usage metadata to include reasoningTokens, especially for models like Sonar Deep Research.

Use Cases

This would align the SDK's metadata with Perplexity's actual pricing components, makes it easier to reconcile SDK usage with Perplexity's billing statements.

Additional context

No response