Extend max output token resolution so per-model and default values are
honored at every layer when the client omits max_tokens.

Effective priority (client value always wins if present):
  autoselect per-model -> rotation per-model -> provider per-model
  -> provider default -> rotation default -> autoselect default

- rotation path: consult the selected provider's config (new
  RotationHandler._get_provider_config) so a provider default_max_tokens
  applies even though rotations can't configure max_tokens themselves
- AutoselectModelInfo.max_tokens: new per-model override field
- AutoselectHandler._apply_autoselect_max_tokens applied in both the
  streaming and non-streaming dispatch: per-model override set directly
  (highest), autoselect default threaded via _autoselect_default_max_tokens
  as a lowest-priority fallback
- rotation and both RequestHandler injections consume the threaded
  autoselect default, covering autoselect->rotation and
  autoselect->provider/model dispatch routes
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>