feat: extended token usage

[lgrammel]

Background

Providers have added extended usage information over the past year, and current usage calculations are inconsistent (see #9921 ).

Summary

• restructure LanguageModelV3Usage in specification to include caching, reasoning, and raw usage.
• expose additional inputTokenDetails and outputTokenDetails in LanguageModelUsage
• deprecate reasoningTokens and cachedInputTokens in LanguageModelUsage

Manual Verification

• run examples/ai-core/src/generate-text/openai.ts
• run examples/ai-core/src/stream-text/openai.ts

Tasks

• spec
• ai return
• provider implementation
• model v2 adapter generate
• model v2 adapter stream
• docs
• migration guide
• changeset

Future Work

• add codemod for rewriting access to deprecated LanguageModelUsage properties
• ensure that raw anthropic usage information is correctly forwarded
• language model v2 mapping: map stream parts as needed

Related Issues

Resolves #9921