Provider-level defaults were ignored when a provider had no per-model
configs, so coderai/broker providers reported a fixed 4096 context window
and clients fell back to their own defaults.

- model list (handlers.py): fall back to provider_config.default_context_size
  for context_window/context_length before inferring
- rotation/autoselect model list (routes/api.py): attach context_window/
  context_length resolved from rotation/autoselect/provider defaults (was
  absent entirely, causing clients to default to 4096)
- handle_rotation_model_list: honor provider default_context_size before
  heuristic auto-derivation
- max output tokens: add ProviderModelConfig.max_tokens and
  default_max_tokens on Provider/Rotation/Autoselect configs, plus
  get_max_completion_tokens_for_model resolver, applied as a fallback in
  all request paths when the client omits max_tokens
- get_max_request_tokens_for_model: fall back to
  provider_config.default_max_request_tokens; drop duplicated dead block
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>