ladder: post-merge follow-ups from #1362 pre-merge review (multi-account test gap + nits)

[cristim]

Non-blocking findings from the final pre-merge adversarial review of #1362 (merged as 7adacd7):

1. Test gap (LOW): no positive test asserts SkippedMultiAccount==1 - only ==0 assertions exist (handler_ladder_test.go ~889). The code path (handler_ladder.go ~232-234, configs whose cloud account does not match the resolved caller account) is real but unexercised. Add a one-config mismatched-ExternalID test asserting the config is visibly counted skipped, nothing persisted for it, and other configs still process (isolation).
2. NIT: handler_ladder.go ~256 variable cap shadows the builtin; rename to capability.
3. NIT: handler_ladder.go ~437-439 comment says the unknown-cadence error surfaces "during Allocate"; it actually surfaces in ladderConfigToEngine (still pre-persist, fail-loud). Fix the comment.

Small, single-PR scope. Part of LeanerCloud/cloud-commitments-platform#70 / tracker LeanerCloud/cloud-commitments-go#27.

[cristim]

Implementation is up as PR #1371 (branch fix/ladder-1370-followups): positive multi-account skip test (fails when the account comparison is inverted, proving the invariant), cap->capability rename, and the corrected cadence comment. Gates green (build/vet 0, internal/server 402 tests, gocyclo clean). Awaiting CodeRabbit, then it merges in the queue.