fix(t8n): serialize receipt root/status as mutually exclusive

[hudem1]

Refs #12835 (partial — see below)

Changes

• TxReceiptConverter.Write now emits either the post-state root (pre-Byzantium, when receipt.Root is set) or the status code (post-Byzantium), instead of always writing both with root defaulting to the zero hash. This matches EIP-658, the receipt schema in ethereum/execution-apis, and go-ethereum's marshalReceipt.
• Added two regression tests covering the pre- and post-Byzantium cases.

Context

Issue #12835 reports (auto-generated) that many JSON-RPC fields serialize as null while the docs type them as non-null. After comparing each item against the spec (ethereum/execution-apis) and go-ethereum, 11 of the 12 items are not defects — the null values are intentional and spec-/Geth-conformant (pending blocks/transactions, to: null for contract creation, conditional trace fields). Those are at most a docs-site annotation matter and are not addressed here.

The one genuine code-level finding is the receipt root/status handling, and even there the user-facing JSON-RPC methods (eth_getTransactionReceipt, eth_getBlockReceipts, parity_getBlockReceipts, proof_getTransactionReceipt) were already correct: they serialize the ReceiptForRpc DTO, which omits the null field via JsonIgnoreCondition.WhenWritingNull.

TxReceiptConverter is the standalone converter for bare TxReceipt, used by the t8n tool (tools/Evm/T8n). It emitted both root and status, so t8n result.receipts carried a spurious "root":"0x00…00" alongside "status" for post-Byzantium receipts (t8n forces PostTransactionState = null for successful txs). This PR aligns that output with Geth's evm t8n. No JSON-RPC response changes.

Types of changes

• Bugfix (a non-breaking change that fixes an issue)

Testing

Requires testing

• Yes

If yes, did you write tests?

• Yes

Notes on testing

Added Post_byzantium_receipt_serializes_status_without_root and Pre_byzantium_receipt_serializes_root_without_status in ReceiptsForRpcTests. Full ReceiptsForRpcTests and TransactionReceiptsSubscription suites pass.

Documentation

Requires documentation update

• No

Requires explanation in Release Notes

• No

Remarks

Scope is intentionally limited to the t8n output path; the remaining #12835 items are behavior-correct and, where actionable, belong in the docs repo (docs are auto-generated via tools/DocGen).