…teness error (#22022)

Two review follow-ups from #22000.

### keys[] existence gate: one pre-block read instead of two

`collectAccessedState` gated each address on `!accountExists(addr) &&
!existedPreBlock(addr)`. When an accessed address exists in neither pre-
nor post-state, `accountExists` runs its own `inner.ReadAccountData` and
returns false, so `&&` does not short-circuit and `existedPreBlock`
reads the same account a second time.

Both are merged into `hasWitnessLeaf`, which reads the inner (pre-block)
reader at most once:

- post-state present (overlay non-nil, not deleted) → include, no inner
read
- otherwise → one inner read decides it

Predicate is unchanged: `hasWitnessLeaf == accountExists ||
existedPreBlock` across the full `(deleted, overlay-present,
overlay-nonnil, inner-exists)` table. `innerExists` is the shared
pre-block primitive; `accountExists` keeps its post-state semantics.

### checkWitnessKeysComplete: bounded error

The completeness check listed every missing preimage in the error
string. Capped at 16 with a `(+N more)` suffix; the leading count stays
the full total. Verification diagnostic only — no returned witness data
changes.

### Tests

- `TestRecordingState_hasWitnessLeaf_SingleInnerRead` — pins the
at-most-one-read property via a read-counting reader.
- `TestCheckWitnessKeysComplete/large missing list truncated` — pins
truncation and preserved total.

Refs
#22000 (comment),
#22000 (comment)

(cherry picked from commit 425af38)