Add hasExitTime and fix error when set state startTime and transition duration is 0

[luzhuang]

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• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

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Summary by CodeRabbit

• New Features

  • Enhanced handling of animation transitions with improved exit time logic.
  • Introduced hasExitTime property for more nuanced control over transitions.
  • Added support for solo state tracking in transitions for better animation management.
  • Integrated a new TransitionSource class for improved transition management.

• Bug Fixes

  • Streamlined calculations for playCostTime to ensure accuracy during playback.
  • Removed outdated calculations for transition times in tests.

• Documentation

  • Updated comments to clarify the behavior of transition methods and properties.

• Tests

  • Added new test cases to validate animator state transitions and exit timing behavior.