Cap VectorFstImpl::kMaxStates by StateId and validate start state before narrowing.

[copybara-service]

Cap VectorFstImpl::kMaxStates by StateId and validate start state before narrowing.

In VectorFstImpl<S> (lib/vector-fst.h):

1. StateId (typename S::StateId) is int32_t for StdArc (max 2^31 - 1),
   whereas kMaxStates was 1LL << 52. If hdr.NumStates() (or the actual
   stream state count when hdr.NumStates() == kNoStateId) exceeded
   std::numeric_limits<StateId>::max(), the StateId state loop counter in
   Read() would overflow signed StateId.
2. In VectorFstImpl::Read, hdr.Start() (int64_t) was passed to
   impl->BaseImpl::SetStart(hdr.Start()) (StateId) before the post-loop
   start-state range check, so a 64-bit value such as 1LL << 32 truncated to
   0 in 32-bit StateId and passed validation on any non-empty FST.

Fix by:

• Capping VectorFstImpl::kMaxStates at
  std::min<int64_t>(0x10000000000000LL, std::numeric_limits<StateId>::max()).
• Validating hdr.Start() as int64_t in VectorFstImpl::Read before calling
  SetStart, and guarding state == kMaxStates in the state-reading loop when
  hdr.NumStates() == kNoStateId.
• Expanding VectorLimitsTest in fst_limits_test.cc to cover 64-bit state
  counts and start states that truncate into 32-bit StateId.