`eth_sendRawTransaction`: RLP decode failure returns wrong JSON-RPC error code

[manusw7]

Description

Every RLP decode failure on eth_sendRawTransaction is reported with code -32000 (InvalidInput) and the literal message "Invalid RLP.", regardless of the actual cause. Per the JSON-RPC 2.0 spec, Reth, and Besu, parameter-validation failures (which malformed RLP is) should be -32602 (InvalidParams). Canonical Geth + Erigon currently emit -32000 for this axis; we deliberately diverge from them to align with the spec.

This supersedes #11206 (closed; PR #11325 never merged).
The original closing PR hard-coded a single Geth string that no longer matches any current Geth response — the right fix is the code change plus a Geth-inspired but not byte-identical message.

Input	NM	Geth
"0xc0"	-32000 / "Invalid RLP."	-32000 / "rlp: too few elements for types.LegacyTx"
"0xd4"	-32000 / "Invalid RLP."	-32000 / "rlp: value size exceeds available input length"
"0x"	-32000 / "Invalid RLP."	-32000 / "typed transaction too short"
"0x09c0" (unknown type)	-32000 / "Invalid RLP."	-32000 / "transaction type not supported"
"0x03c0" (EIP-4844)	-32000 / "Invalid RLP."	-32000 / "unexpected EOF"
"0x04c0" (EIP-7702)	-32000 / "Invalid RLP."	-32000 / "rlp: too few elements for types.SetCodeTx"

Note: NM and canonical Geth currently agree on the code (-32000); only the message differs. Public Geth-labelled providers tested (Infura, publicnode, 1rpc.io) all wrap this to -32602/-32600 at their edge. The fix proposed here moves NM to -32602 to align with spec + Reth + Besu

13 distinct inputs in the rpc-tests fixture PR corpus (test_01-06, 12-16, 20, 21) reach the same code path and all collapse to the same vague NM response.

Steps to Reproduce

{
  "jsonrpc": "2.0",
  "method": "eth_sendRawTransaction",
  "params": ["0xd4"],
  "id": 1
}

Expected

-32602 rejection, with a descriptive message (Geth-inspired but not required to match byte-for-byte, see this PR for more).

Actual (Nethermind v1.38.0+c07a4d65)

{"jsonrpc":"2.0","error":{"code":-32000,"message":"Invalid RLP."},"id":1}

Fix Locations

• src/Nethermind/Nethermind.JsonRpc/Modules/Eth/EthRpcModule.cs:369-372 — the catch (RlpException) block at :369 (the Fail("Invalid RLP.", ErrorCodes.TransactionRejected) is at :371). Change ErrorCodes.TransactionRejected → ErrorCodes.InvalidParams; replace the literal "Invalid RLP." with "transaction could not be decoded: " + e.Message so the underlying RLP cause comes through.
• src/Nethermind/Nethermind.JsonRpc.Test/Modules/Eth/EthRpcModuleTests.cs:1908-1914 — Send_raw_transaction_returns_invalid_rlp_for_empty_list currently pins the old code/message via Is.EqualTo; switch to Does.Contain against the new prefix.
• EthRpcModuleTests.cs:1916-1931 — SendRawTransactionSyncFailureCases source (:1929-1931) yields TestCaseData("c0", null, ErrorCodes.TransactionRejected, "Invalid RLP") for the sync variant; update to InvalidParams + new fragment.

Notes

• rpc-tests fixtures test_01-06, 12-16, 20, 21 in PR pin {"code": -32602, "message": null} via the new per-fixture code-only sentinel. 13 fixtures currently FAIL against NM; all 13 flip to PASS when this fix lands.