eth: default Block param to latest on state methods — implementation + conformance tests (builds on #812)

[MysticRyuujin]

Summary

Builds on #812 by @manusw7 and carries it through to a tested, client-verified state:

1. Spec (src/eth/state.yaml): mark the Block parameter required: false with description: "default: 'latest'" on the six state-reading methods (eth_getBalance, eth_getStorageAt, eth_getStorageValues, eth_getTransactionCount, eth_getCode, eth_getProof) — identical to @manusw7's proposal in Default Block param to 'latest' on eth_* state methods #812.
2. Conformance tests (tools/testgen/generators.go): add a *-default-block case to each method that omits the Block parameter and asserts default-to-latest behavior.
3. Fixtures (tests/**/*.io): generated via make fill against a go-ethereum build that implements the behavior.

This addresses the two open items in #812's checklist: a go-ethereum implementation, and hive-enforceable conformance tests for the default-to-latest behavior.

Credit

The proposal, rationale, and spec change originate with @manusw7 in #812. This PR exists to implement and test it across clients — full credit to @manusw7 for the standards work.

Validation

• make build regenerates openrpc.json with Block required: false on all six methods.
• ./tools/speccheck -v passes. Negative-tested: flipping a method back to required: true makes speccheck reject the omitted-block fixture (missing required parameter), confirming the spec change is load-bearing.
• The six fixtures replay green in hive rpc-compat against a go-ethereum build with the implementation.

Cross-client status

• go-ethereum: implemented; passes all six. (PR linked below.)
• Nethermind: defaults to latest for five of six already; eth_getStorageValues required a one-method fix (its block param was non-nullable). (PR linked below.)

Related

• Source proposal: Default Block param to 'latest' on eth_* state methods #812 (by @manusw7)
• go-ethereum implementation: internal/ethapi: default block parameter to latest on state methods go-ethereum#35100
• Nethermind eth_getStorageValues fix: JsonRpc: default omitted block to latest for eth_getStorageValues NethermindEth/nethermind#11883
• Erigon implementation: rpc/jsonrpc: default omitted block to latest on state methods erigontech/erigon#21586
• Besu implementation: Default omitted block parameter to latest on eth state methods besu-eth/besu#10587
• ethrex implementation: feat(l1): default omitted block to latest on eth state methods lambdaclass/ethrex#6780

CI note (expected red until go-ethereum merges)

The Test job's make fill step is expected to fail for this PR, and that is
structural — not a defect:

• make fill regenerates fixtures by building the geth pinned in tools/go.mod.
• The new *-default-block fixtures require a geth that defaults an omitted block
  to latest — i.e. the accompanying go-ethereum PR (internal/ethapi: default block parameter to latest on state methods go-ethereum#35100),
  which isn't merged yet. Upstream-pinned geth still errors on the omitted block,
  so make fill can't reproduce the committed fixtures and the "no untracked /
  changed files" gate fails.
• make test (speccheck) and make lint pass; the fixtures were generated with a
  local geth carrying the change and validate against the updated spec.

This resolves once go-ethereum#35100 merges and this PR's tools/go.mod is bumped
to that commit. (A replace to a personal fork would make CI green but isn't
mergeable, so it's intentionally not committed.) This is the two-PR ordering
#812 anticipated.