Avoid caching sync error and interrupt writes

[Alexxigang]

Summary

Fixes a sync-only cache inconsistency where SyncPregelLoop.put_writes() cached ERROR and INTERRUPT task writes even though the async path already skips them.

That could poison the sync cache with failed/interrupted task results. In practice, repeated invocations could replay cached failures instead of re-running the task, and cached interrupt() writes could be replayed into the sync scheduler and crash with IndexError: deque index out of range.

Changes

• add the same ERROR / INTERRUPT cache guard to SyncPregelLoop.put_writes() that already exists in AsyncPregelLoop.put_writes()
• add targeted regression coverage for sync and async task caching behavior
• verify that:
  • error writes are never cached
  • interrupt writes are never cached
  • successful writes are still cached
  • cache keys remain input-specific for successful writes

Testing

python -m py_compile libs/langgraph/langgraph/pregel/_loop.py libs/langgraph/tests/test_sync_cache_guard.py
python -m pytest --noconftest libs/langgraph/tests/test_sync_cache_guard.py -q

Passed locally:

• 8 passed in 0.23s

Issue

Closes #7015