fix: sync lastLog cache when append config processing fails

[anyasabo]

Problem

appendEntries can store new entries durably, then fail while processing configuration entry decoding, and return without updating cached lastLog.

Result: durable log advances, but r.getLastLog() remains stale.

How this can happen in concrete terms

1. Leader sends AppendEntries containing at least one config-changing entry.
2. Follower successfully writes new entries via StoreLogs.
3. Config-processing step fails (e.g., malformed peers/config payload in the new entry).
4. Function returns error before cached lastLog update step.

This produces a cache/store mismatch even though append persistence already succeeded.

Mitigations that help

• Avoid malformed config-entry payloads (transport hygiene, tooling checks).
• Ensure config-change producers are well-validated.

But pre-fix, once this error path was hit, cache divergence still occurred.

Impact

Availability/liveness degradation risk after an error event:

• stale replication metadata,
• incorrect follow-up consistency checks/retry behavior,
• prolonged instability after malformed config-entry events.

How we would notice in production

• Warning logs: failed to append entry ... failed to decode peers (or decode config).
• Replication behavior inconsistent with actual durable index.
• Possible mismatch between debug/stats last_log_* and on-disk state immediately after failure.

Provenance / Preconditions

• Not a brand-new regression: this specific return path was introduced with config-processing integration (e59f65d6, 2021).
• Trigger requires:
  • append path where StoreLogs succeeds,
  • subsequent config-entry decode failure in processing loop.

What this PR changes

• On config-processing failure after append, updates cached lastLog to the newly stored last entry before returning.
• Adds deterministic regression test covering store-success + config-decode-failure path.

Reviewer reproduction (live in-process path)

Reproduce pre-fix behavior

1. Checkout parent commit:
   • git checkout 53b8474^
2. Bring this PR's regression test into that tree:
   • git checkout fix/append-config-process-failure-lastlog -- raft_test.go
3. Run:
   • go test -run "TestRaft_AppendEntriesConfigProcessFailureRefreshesLastLog" -count=1 .
4. Expected pre-fix: stale cached lastLog assertion failure.

Verify fixed behavior

1. Checkout this branch (fix/append-config-process-failure-lastlog).
2. Run:
   • go test -run "TestRaft_AppendEntriesConfigProcessFailureRefreshesLastLog" -count=1 .
3. Expected: pass; cached lastLog aligns with durable store after error.

Test plan

• go test -run "TestRaft_AppendEntriesConfigProcessFailureRefreshesLastLog" -count=1 .
• go test -run "TestRaft_AppendEntry$|TestRaft_AppendEntriesConfigProcessFailureRefreshesLastLog" -count=1 .