fix(combo): name output_tokens as the exclusion reason instead of structured output (#12374)

When every combo target is excluded because the request's max_tokens exceeds each target's known output limit, the terminal 400 now says so — requested max_tokens against the pool's highest known ceiling — instead of the unrelated "supports structured output for this request". Diagnostics (unmet, excluded[].reason, terminalReason) are unchanged; only the message for the output_tokens primary reason moves.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.