Block object contract drift: required difficulty/totalDifficulty fields are omitted

[roninjin10]

Summary

Block object serialization omits required difficulty and totalDifficulty fields in normal dev-node operation.

Why this matters

This is a schema-level contract drift for eth_getBlockByNumber / eth_getBlockByHash. Consumers expecting required fields can break.

Normative contract

• docs/specs/json-rpc-contract.md:355-356 marks difficulty and totalDifficulty as required non-null QuantityHex fields for block objects.

Evidence

Block conversion conditionally null/omits both fields:

• src/rpc/handlers/block_query_handlers.zig:353-354

.difficulty = if (resp.difficulty > 0) try quantityFromU256(allocator, resp.difficulty) else null,
.totalDifficulty = if (resp.totalDifficulty) |td| try quantityFromU256(allocator, td) else null,

Mined block header currently sets difficulty to zero:

• src/rpc/handlers/tx_submission.zig:695

header.difficulty = 0;

With the current conversion logic, this causes difficulty to be omitted/null instead of required "0x0".

Suggested scope

• Emit difficulty and totalDifficulty unconditionally as QuantityHex per contract.
• Ensure zero values are serialized as "0x0", not omitted.
• Add regression tests for both genesis and mined blocks asserting presence and shape of these fields.