Add a CallbackModeDiscrete property to AnimationMixer to handle the case of blending Continuous and Discrete tracks

[TokageItLab]

• Closes Add CallbackModeDiscrete to AnimationMixer and improve the setting method for UpdateMode in AnimationTrackEditor godot-proposals#8512
• Closes The animation_mixer gives a false warning about the value track type #83545

As noted in godotengine/godot-proposals#8512, this allows user control how the Discrete/Continuous tracks are handled when blending occurs. See also #86630.

[TokageItLab]

Fixed a bug where INTERPOLATION_NEAREST did not handle reverse playback correctly (the bug existed before this PR). This ensures consistency in playback when converting from Discrete.

However, the old Nearest interpolation is not unreasonable, so for now this is only used when converted from Discrete by Force Continuous option. If necessary, we can add it to Animation::InterpolationType can be added later (as like INTERPOLATION_NEAREST_BIDIRECTIONAL).

[lyuma]

After Tokage explained it to me, I understand what this is fixing and it makes sense.

I do think users are going to be confused, so I'm suggesting some documentation changes.

[akien-mga]

Thanks!