test(rpc): cover lightwalletd block RPCs

[cleanerzkp]

Motivation

Add the low-cost lightwalletd gRPC coverage requested in #11224 and scoped by the Zebra team response.

This draft is stacked on #10953 because the in-process server harness is currently on lightwalletd-grpc.

Solution

Add test-only coverage through the existing in-process gRPC server for:

• GetLatestBlock returning the mocked best tip height and hash;
• inclusive ascending and descending GetBlockRange order;
• a range crossing the available state, which returns prior blocks and then NotFound;
• an unspecified BlockID, which returns InvalidArgument without a state read.

Stream reads use bounded waits so a failed server response cannot hang the tests.

Tests

Passed locally:

• cargo test -p zebra-rpc lightwalletd::tests::vectors -- --nocapture — 7 passed;
• cargo fmt --all -- --check;
• cargo clippy --workspace --all-targets -- -D warnings.

The full cargo test --workspace run reached zebra-network, then stopped on five unrelated macOS environment failures: three tests could not bind 127.0.0.2, and two peer-cache tests could not use the default cache path. The modified zebra-rpc suite passed. Local validation used installed stable Rust 1.95 because repeated downloads of the pinned Rust 1.91 toolchain reset at static.rust-lang.org; CI should provide the authoritative pinned-toolchain result.

Specifications & References

• Refs Test coverage for the lightwalletd CompactTxStreamer gRPC server #11224
• Depends on feat(rpc)!: add lightwalletd-compatible gRPC server implementing CompactTxStreamer #10953

Follow-up Work

After #10953 merges, rebase this branch onto main, retarget the PR, and rerun the required gates.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: OpenAI Codex helped inspect repository guidance, implement the focused test coverage, run validation, and review the diff.

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.

[oxarbitrage]

Reopened — this was auto-closed when its base branch (#10953's lightwalletd-grpc) merged, not a review outcome. #10953 is now on main (78acfd5) and this PR is retargeted to main, so it needs a rebase; the coverage is still wanted (tracked in #11224). @cleanerzkp, can you rebase — or let us know if you'd rather we pick it up?