- FakeClient: an Integer list_size is now an exact length (Range still
  randomizes) — the default 1..3 Range is unaffected.
- Codegen flags two variables that underscore to the same kwarg
  ($userId + $user_id) instead of silently dropping one on the wire.
- Faraday transport raises instead of silently ignoring headers:/block
  when passed a prebuilt Connection.

Skipped as low-value: honoring @skip/@include in the fake (harmless,
codegen already nils those), and the explicit-JSON-null-for-optional-input
gap (an inherent T::Struct nil-vs-absent limitation, not cleanly fixable).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>