…nal messages

- Context window (pullfrog #2, load-bearing): getModelContextWindow() was
  called without args and always returned the 120K default, so the 30%
  oversized threshold was hardcoded at ~36K regardless of the actual
  model. Added currentModelSettings to AgentRunContext, stashed after
  configureModelSettings, and read lazily inside toModelOutput.
- Compression prompt (pullfrog #4, load-bearing): buildCompressPrompt
  only kept role==='system' messages, dropping the original user query
  and conversation-history prefix. Now takes originalMessageCount and
  preserves messages.slice(0, originalMessageCount) as the prefix —
  matching the pre-middleware handlePrepareStepCompression behavior.
- Async-iterator fallback (pullfrog #1): replaced the unsound
  `as unknown as AsyncIterator` cast with a proper Reader → iterator
  adapter so the dead branch is safe if ever triggered.
- Middleware spec-version comment (pullfrog #5): documented which
  @ai-sdk/provider versions the wrapGenerate/wrapStream contract was
  verified against.
- JSON round-trip (pullfrog #3): kept as-is. The round-trip is not a
  no-op — it launders `unknown` tool args through JSONValue and strips
  non-JSON types. Added a comment explaining this.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>