refactor: NetworkAnimator cleanup and code quality improvements (#4001)

  refactor: cleanup and code quality improvements in NetworkAnimator
  - Add readonly to fields where applicable
  - Narrow internal fields and methods to private where applicable
  - Fix naming casing (TransitionStateinfo, m_DestinationStateToTransitioninfo, field prefixes)
  - Replace ContainsKey + Add/index lookups with TryAdd/TryGetValue
  - Remove unused variables and dead code
  - Improve XML summaries and comment clarity

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Co-authored-by: Emma <emma.mcmillan@unity3d.com>

Author: noellie-velez