feat(jsonrpc): eth-compatible JSON-RPC server bootstrap

[libotony]

Description

Bootstrap of an Ethereum-compatible JSON-RPC 2.0 server for Thor.

This PR lands the server engine and wiring only — a minimal, self-contained JSON-RPC core plus two example namespaces to prove the full request path end to end. It is intentionally small; the complete Ethereum method set, websocket subscriptions, filters and write paths are out of scope and tracked as follow-ups.

What's in it

Reflection-based registry (api/jsonrpc/server) — a from-scratch JSON-RPC 2.0 core, no dependency on go-ethereum/rpc. It reflects over a service struct's exported methods and exposes each as <namespace>_<method>, following go-ethereum's rpc/service.go contract:

• Legal signatures: Method() error, Method(args...) Result, Method(args...) (Result, error), Method(ctx, args...) (Result, error).
• A leading context.Context is injected by the server and is not part of the JSON params.
• Positional params: a pointer arg is optional (nil when omitted), a non-pointer arg is required (-32602 if missing); omission is tail-only.
• Duplicate method registration returns an error rather than silently overwriting.

HTTP transport (server/http.go) — POST /rpc, single request and batch array, notification handling (no id → no reply, per spec), batch-size cap.

Error model (server/json.go) — standard codes (-32700/-32600/-32601/-32602/-32603/-32000) plus a DataError interface so business errors can attach structured error.data. A panic inside a method is recovered at the call layer and returned as -32603 instead of tearing down the connection.

Package layout — three packages with a clear dependency direction:

• api/jsonrpc/server — the transport-agnostic engine (stdlib only).
• api/jsonrpc/service — thor business namespaces (eth, net).
• api/jsonrpc — top-level composition + Mount.

Example methods — eth_chainId, eth_blockNumber, net_version. Enough to exercise reflection, dispatch, batching and error codes; not a complete namespace.

Wiring — a new --enable-rpc flag mounts the server under /rpc on the existing REST router, reusing thor's middleware chain. Off by default.

Dependencies

Stacked on top of eth-equivalence (the PR base). Should be reviewed/merged after it.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests cover the reflection contract (signature acceptance/rejection, ctx injection, required vs optional args), dispatch and every error code, batch and notification handling, and the two example namespaces against a genesis-only testchain.

• go test ./api/jsonrpc/...
• go build ./...
• golangci-lint run + modernize clean

Test Configuration:

• Go Version: 1.26.5

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my feature works
• New and existing unit tests pass locally with my changes
• I have not added any vulnerable dependencies to my code