fix: Support previous v4 of Vercel AI SDK

[jsonbailey]

fix: Fix metric tracking for v5 responses

---

Note

Add v5 token usage mapping (inputTokens/outputTokens with precedence) and switch to LanguageModel API; remove @ai-sdk/provider dependency.

• Server AI Vercel Provider:
  • Metrics: createAIMetrics now supports v5 fields inputTokens/outputTokens (prefers them over v4 promptTokens/completionTokens), with totalTokens and null-safe defaults.
  • API Update: Replace LanguageModelV2 with LanguageModel; update generateText usage and getters accordingly; clean up dynamic imports (remove as any).
• Tests:
  • Add cases for v5 token fields and precedence over v4 when both present.
• Dependencies:
  • Remove @ai-sdk/provider from dev and peer dependencies.

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[github-actions]

@launchdarkly/browser size report
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Uncompressed size: 789399 bytes

[github-actions]

@launchdarkly/js-sdk-common size report
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Uncompressed size: 122411 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
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Compressed size limit: 25000
Uncompressed size: 74698 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 17636 bytes
Compressed size limit: 20000
Uncompressed size: 90259 bytes

[jsonbailey]

The build issues should be solved by #963 and once it is merged I will update this branch.