error-groups, tools, tests: add spec-mandated error-code fixtures for methods

[simsonraj]

Follow-up to #650 that turns the new error-code catalog into a forcing function for client adoption via hive's rpc-compat simulator.

• Adds code 1002 "Replacement transaction underpriced" to txpool-errors.yaml.
• Adds ExpectErrorCode to the testgen. When set, rpctestgen rewrites the captured error.code in the generated .io fixture to the spec-mandated value
• Adds eth_sendRawTransaction error scenarios, each tagged with its catalog code.
• Extends speccheck to validate each fixture's error.code against the method's resolved errors[]; emits [WARN] on unknown codes.

Scenarios

Fixture	Spec code	Group
send-nonce-too-low	1	ExecutionErrors
send-intrinsic-gas-too-low	800	GasErrors
send-tx-gas-exceeds-block-limit	803	GasErrors
send-tip-above-fee-cap	804	GasErrors
send-insufficient-funds	809	GasErrors
send-already-known	1000	TxPoolErrors
send-replacement-underpriced	1002	TxPoolErrors

Measured on docker-ized go-ethereum, hyperledger/besu, nethermind/nethermind (full data and fixtures: simsonraj/eth-err-tests).

Deferred

Four scenarios that can't yet be reliably reproduced across all clients because hive doesn't expose the relevant client-launch flags. (commented currently)

[MysticRyuujin]

From RPC Standards call: Ideally, we need to get a go-ethereum PR to fix, so we can remove the error code overwriting logic from the generator, and then update the go-ethereum target.