fix: add null safety to FileDiff getBefore/getAfter

[yangyu0227]

Root cause: SseEventListener's Gson instance does not register FileDiffDeserializer, so when the server sends SSE diff events with null 'before'/'after' values, Gson creates FileDiff instances with null fields (bypassing constructor via UnsafeAllocator).

Fix:

1. Register FileDiffDeserializer in SseEventListener's Gson builder (prevents null fields at the source)
2. Add null-safe elvis operator in SessionManager.resolveBeforeContent (defense in depth)