Improve EMA documentation clarity and stability test coverage

[savirpatil]

This PR improves documentation clarity and test coverage for ExponentialMovingAverage in dm-haiku.

Changes include:

• Updated a doctest example in moving_averages.py for clearer and more explicit usage

  • adds explicit imports for clarity
  • introduces a named RNG variable for readability
  • preserves the existing seed value
  • improves example completeness and readability

• Added a dtype stability test for repeated EMA updates

  • ensures float32 inputs remain float32 across repeated applications

• Added a numerical stability test for repeated constant-input EMA updates

  • verifies outputs remain finite during repeated updates
  • validates stable bounded behavior under iterative application

No functional changes to library behavior are introduced.