Stateless client: failed to fetch light block due to missing signed header

[martintomazic]

Follows #6527.

Problem

Using stateless client node, I run raw sgx rofl on the testnet and encountered the following issue:

{"err":"verification: StringTracer: io error: rpc error: server error: module: unknown code: 1 message: failed to fetch light block for height: 32140178: missing signed header","level":"error","module":"runtime/consensus/cometbft/verifier","msg":"Consensus verifier terminated, aborting","ts":"2026-05-18T22:30:08.489972817Z"}

^^ happens for the ronl component, at the tip of the chain repeatedly.

Possible solution

Stateless client is receiving notification about new blocks from its provider, triggering RHP consensus sync. Then the runtime enclave fetches light block using verifier io, that fetches this block via host from the remote peer, that doesn't have this block available.

On the remote peer light block may not be available due to the following reasons:

1. Remote peer hasn't received light block yet it only has validator set available for the given height (future height).
2. Light block is already available (i.e. commit is already part of the blockstore), however mux is still processing, so we treat it as a future height/locally not available (probably a bug, can be improved).

See code: https://github.com/oasisprotocol/oasis-core/blob/b6a87ba24f370845617840a085557ca3c7fd354c/go/consensus/cometbft/full/common.go#L610:L632

[martintomazic]

@peternose I forgot why we have to maintain GetLightBlocks with empty header? There is part of the code that is abusing this to get future validators? Could we fix that code to start calling GetValidators, which would allow us to fix GetLightBlock to only return light block if it actually exists.