test(zebra-rpc): assert submitblock serializes an accepted block to result: null

[natalieesk]

Motivation

Closes #10571.

A report claimed accepted submitblock responses omit the JSON-RPC result member (so a strict client sees {"jsonrpc":"2.0","id":"submitblock"} with no result). It is not reproducible: SubmitBlockResponse::Accepted is a unit variant under #[serde(untagged)], which serializes to an explicit null, so the response carries "result": null. The reported omission was almost certainly on the client side (the Stratum bridge). This PR locks the correct behaviour in with tests so the issue can be closed as not-reproducible.

Solution

Add unit tests in zebra-rpc/src/methods/types/submit_block.rs: an accepted response serializes to serde_json::Value::Null, and a non-accepted response serializes to its kebab-case string reason ("rejected"), matching the Zcash RPC contract. No behaviour change — tests only.

Tests

cargo test -p zebra-rpc --lib -- methods::types::submit_block::tests
test result: ok. 2 passed

cargo fmt and cargo clippy -p zebra-rpc --lib --all-features are clean.

Specifications & References

zcashd submitblock contract: acceptance is a null result, rejection is a string reason (https://zcash.github.io/rpc/submitblock.html).

Follow-up Work

None. The issue is not-reproducible and can be closed once this merges.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: Claude (Claude Code) — wrote the tests and this description.

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.

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