refactor(consensus): separate block and mempool transaction verification

[syszery]

Motivation

This PR makes progress toward #10131 by separating the block and mempool transaction verification paths inside Verifier<ZS, Mempool>.

The existing Service::call() implementation interleaved block and mempool verification logic throughout a single async block, branching on req.is_mempool() at multiple points. Separating the two paths makes each one easier to follow independently and should make any future architectural split simpler.

Solution

This PR consists of three related refactors:

1. Split spent_utxos into block_spent_utxos and mempool_spent_utxos

2. Split Service::call into verify_block and verify_mempool

   call() now dispatches immediately to one of two private methods. Each method contains only the logic for its respective verification path, eliminating the internal is_mempool() branching. Shared validation steps remain the same, while block- and mempool-specific behavior is handled directly within each method.

3. Replace &Request in check_maturity_height with explicit arguments

   check_maturity_height previously accepted &Request only to extract the transaction, height, and known_utxos. Since the function is only used during mempool verification, where Request::known_utxos() always returns an empty map, it now accepts only the values it actually needs (tx, height, and spent_utxos).

These refactors are intended to preserve the existing verification behavior while making the two verification paths easier to understand and modify independently.

Tests

• Existing transaction verification tests pass for both block and mempool verification paths.
• The refactor is intended to preserve behavior; no new functionality is introduced.

Follow-up Work

This PR intentionally keeps the existing Verifier<ZS, Mempool> type and public API unchanged. With the verification logic now isolated into verify_block and verify_mempool, a future refactor could split these into separate verifier types if that is still the desired direction for #10131.

Question for reviewers: Is the intended resolution of #10131 to introduce separate verifier types, or is the current separation of the internal verification paths sufficient? I'm happy to continue with a follow-up refactor—or extend this PR—depending on the preferred direction.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: Claude for refactoring discussion and code review; ChatGPT for independent review and sanity checking.

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.

[Copilot]

Pull request overview

This PR is an internal refactor of the Zcash transaction verifier in zebra-consensus. It splits the single Service::call implementation of Verifier<ZS, Mempool> — which previously interleaved block and mempool logic and branched on req.is_mempool() throughout one async block — into two dedicated private methods, verify_block and verify_mempool. It makes progress toward #10131 (eventually splitting the verifier into two service implementations) without changing the public Verifier type, request/response API, or verification behavior.

Changes:

• call() now dispatches by request variant to verify_block / verify_mempool, each containing only its path's logic (block uses block time + AwaitUtxo; mempool uses median-time-past, UnspentBestChainUtxo + AwaitOutput, maturity check, and the nullifier/anchor check).
• spent_utxos is split into block_spent_utxos (known_utxos + AwaitUtxo) and mempool_spent_utxos (best-chain + mempool fallback).
• check_maturity_height now takes explicit tx/height arguments instead of &Request, hardcoding an empty known_utxos map (equivalent for the mempool-only call site).

Review notes (per Zebra guidelines)

• NITPICK: A relocated TODO in verify_mempool still references the old spent_utxos() function name (now mempool_spent_utxos()).
• Process: This touches consensus-critical verification. Behavior appears faithfully preserved (check ordering, coinbase handling, block vs. mempool lock-time, omission of maturity/anchor checks in the block path, mempool UTXO fallback, and spent_mempool_outpoints propagation all match the prior logic). The PR description's "discussed in an issue/with the team" and "changelog/docs up to date" checkboxes are unchecked; for a behavior-preserving internal refactor a changelog entry is likely unnecessary, but the consensus surface warrants careful human verification.