feat: Add toVercelAISDK method to support easy model creation

[jsonbailey]

feat: Add support for tracking streaming text metics with getAIMetricsFromStream
feat: Renamed createAIMetrics to getAIMetricsFromResponse (previous method is marked as deprecated)
fix!: VercelProvider now requires type safe parameters for Vercel models
fix: Properly convert LD model parameters to Vercel model parameters
fix: Prefer totalUsage over usage when mapping to LDTokenUsage
fix: Check finishReason for an error when determining model success

---

Note

Adds toVercelAISDK, enforces typed Vercel model parameters with proper LD→Vercel mapping, and enhances usage/streaming metrics handling.

• VercelProvider:
  • Add toVercelAISDK() to build Vercel AI SDK config (merges messages, maps params).
  • Enforce type-safe model parameters via VercelAIModelParameters; update constructor and create() to use mapParameters().
  • Implement mapParameters() to convert LD params (e.g., max_tokens, stop) to Vercel fields (maxTokens, stopSequences).
  • Improve metrics: add mapUsageDataToLDTokenUsage(), support totalUsage/usage, incorporate finishReason, and add createStreamMetrics() for streaming.
• Types/Exports:
  • Introduce types.ts with VercelAIModelParameters, VercelAISDKConfig, VercelAISDKMapOptions, VercelAISDKProvider and export them from index.ts.

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