Poison and evict chain worker on any save failure (linera-io#6053)

## Motivation

Fixes linera-io#6052.

Previously only journal resolution failures poisoned the chain worker.
Other write errors (e.g. possible ScyllaDB timeouts) left the worker
loaded with potentially stale state.

Stacked on linera-io#6051.

## Proposal

- **Poison on any save failure**: `ChainWorkerState::save()` now always
sets `poisoned = true` and returns `WorkerError::PoisonedWorker` on any
error, triggering immediate eviction and a fresh reload on the next
request.

- **Keep `must_reload_view`** for metrics and as way to document that
journaling really requires a notion of view poisoning.

## Test Plan

CI

## Links

- Fixes linera-io#6052
- Stacked on linera-io#6051
- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

linera-core/src/chain_worker/state.rs

@@ -2109,15 +2109,13 @@ where
     ))]
     async fn save(&mut self) -> Result<(), WorkerError> {
         if let Err(error) = self.chain.save().await {
-            if error.must_reload_view() {
-                tracing::error!(
-                    ?error,
-                    chain_id = %self.chain_id(),
-                    "Journal resolution failed; marking worker as poisoned"
-                );
-                self.poisoned = true;
-            }
-            return Err(error.into());
+            tracing::error!(
+                ?error,
+                chain_id = %self.chain_id(),
+                "Chain save failed; marking worker as poisoned"
+            );
+            self.poisoned = true;
+            return Err(WorkerError::PoisonedWorker);
         }
         Ok(())
     }