leios: a synced dingo node can't serve historical endorser blocks to a downstream peer — "endorser block not found" raised as a protocol violation that tears down the NtN connection

[chadaibbhot-git]

Summary

On musashi (magic 164), a dingo relay that has fully synced to tip cannot re-serve
historical endorser blocks to a downstream dingo node syncing from it. When the downstream
requests an EB the relay no longer has, the relay raises protocol error: leios endorser block not found: <slot>.<hash>, which is treated as a protocol violation and tears down the whole
NtN connection — tripping the downstream's short-lived-connection backoff (escalates to ~2m).
Net effect: a downstream dingo node (e.g. an operator's block producer) can't obtain the Leios
region from its own upstream dingo relay; it must fetch EBs from the IOG prototype relay directly.

There are two issues bundled here: (a) the node lacks/doesn't backfill historical EB bodies to
serve them, and (b) a not-found EB is escalated to a fatal protocol violation rather than handled
gracefully.

Current behavior

Both nodes dingo main (cbd517f, incl. #2654), --run-mode leios, core storage. Upstream
synced to tip (~block 121k); downstream a fresh/behind node catching up over NtN. Upstream
(serving) logs, ~6 teardowns in 40 min during catch-up:

ERROR "unexpected connection failure: protocol error: leios endorser block not found: 1893186.057b3488c0f56ffede6f63dd74e4491ba7ebc0785d9adfca21a7772fb2dc58de"
ERROR "unexpected connection failure: protocol error: leios endorser block not found: 1914738.7c62df9b0867b20955f5e14f7445d781ab40c3927b131735c525c5edf83c0455"

Downstream side: connection resets right after selecting the relay as best peer, then
short-lived connection detected, applying backoff (8s → 32s → 2m8s). The downstream still
advances via chainsync between resets (and through EB-light stretches), so catch-up limps along —
but it cannot rely on the relay for the EB region.

Analysis

EBs are served from an in-memory cache; historical EBs the node didn't recently handle are not
present and are not backfilled-to-serve, so the lookup returns not-found. Two problems:

1. Availability/retention: a synced node should be able to serve the EBs of the chain it holds
   to its downstream peers (this is the serving-side mirror of the fetch-side gap in leios: endorser-block backfill stalls a from-genesis sync at the EB-region onset ("leios-fetch served no new transactions within tail budget") #2656).
2. Robustness: a missing EB is escalated to a protocol violation that kills the connection,
   instead of a graceful "don't have it" response — this trips the peer's instability backoff and
   compounds the slowdown.

Distinct from #2630 (verifying dingo serving its own forged EBs) — this is about re-serving
historical EBs already on-chain. Same EB-availability root as #2656 (fetch side).

Reproduction

1. Node A (dingo main, --network musashi --run-mode leios) synced to tip.
2. Node B (empty/behind) syncs from node A over NtN through the Leios region.
3. On node A observe protocol error: leios endorser block not found: <slot>.<hash> and the
   connection torn down; on node B observe repeated short-lived connection detected, applying backoff against node A.

Acceptance

• A synced dingo node can serve historical EBs of its held chain to downstream peers (backfill /
  retain as needed), so a downstream can sync the Leios region from an upstream dingo relay.
• A not-found EB is handled gracefully (no fatal protocol violation / connection teardown).

Code touchpoints

• leios-fetch EB serving / lookup (in-memory cache lookupLeiosEndorserBlock; backfill in
  ouroboros/leios_backfill.go) — origin of leios endorser block not found.
• The path that escalates a not-found EB to a connection-killing protocol violation.
• Related: leios: endorser-block backfill stalls a from-genesis sync at the EB-region onset ("leios-fetch served no new transactions within tail budget") #2656 (fetch/backfill side, same root); not leios: verify endorser block forging and serving end to end as a block producer #2630 (which is forged-EB serving).