debug_executionWitness

During the last call I raised this and was asked to open an issue. I want to spec `debug_executionWitness`, but the four clients that ship it don't agree on what it returns Geth sends headers as JSON objects and leaves keys empty, while Reth, Nethermind, and Erigon (in its new default) send headers as RLP bytes and actually fill the keys in, before I can write the schema, we need to pick one JSON headers or RLP bytes, keys populated or not, and whether we keep a separate `*ByHash` method Geth is dropping theirs, Reth still hasone or just take `BlockNumberOrHash` on a single method. While we are at it, can we also decide whether to require sorted and deduped `state`/`codes` arrays, add a standard error code for when the node can't build a witness something like the existing `4444 Pruned history unavailable`, and either keep or drop Reth's optional `mode` parameter?

Start by comparing the four clients' debug_executionWitness behavior described in the issue, including header encoding, key population, method variants, array ordering, error handling, and the optional mode parameter. Done means the API choices are resolved and documented well enough to define a consistent schema and behavior across clients.

Written by the indexing model from the issue text.