The 200k default dates to the era when 1M context was entitlement-gated
and carried a long-context pricing premium. Anthropic has since made 1M
the default for every model that supports it, with no beta header, tier
gate, or premium. Most users run Opus 4.8 or Fable 5, which are already
1M-default upstream, so the conservative default only added a stale
mismatch for the toggle-capable models.

- Flip the contextWindow default to 1m for Fable 5, Opus 4.6, Sonnet 5,
  and Sonnet 4.6; 200k remains selectable.
- Add resolveClaudeContextWindow so option-less selections resolve to
  the descriptor default instead of implicitly meaning 200k; both
  resolveClaudeApiModelId ([1m] suffix) and the adapter's token
  accounting now share it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>