Fix Android audiobook ended-state parsing for partial locators

[Copilot]

The Android integration job was failing in the audiobook end-of-playback path. The terminal TimebasedState.ended event could carry a currentLocator without progression / totalProgression, and Dart-side normalization crashed while rebuilding Locations.

• Root cause

  • ReadiumTimebasedState.fromJson() normalizes ended-state locators to 1.0 progress.
  • That path called Locator.copyWithLocations() / Locations.copyWith() with partially populated location data and hit a null-unsafe branch.

• Change

  • Reworked shared Locator / Locations copy helpers to preserve existing values via explicit sentinel checks instead of the failing nullable helper path.
  • Keeps ended-state normalization intact: missing progress fields are still normalized to 1.0 when the player reports ended.

• Regression coverage

  • Added a model test for an ended audiobook locator that only includes stable fields such as position and time fragment, with no progression fields.

• Changelog

  • Added an Unreleased entry in flutter_readium_platform_interface/CHANGELOG.md.

final state = ReadiumTimebasedState.fromJson({
  'state': 'ended',
  'currentLocator': {
    'href': 'last.mp3',
    'type': 'audio/mpeg',
    'locations': {
      'position': 3,
      'fragments': ['t=123.45'],
    },
  },
});

expect(state.currentLocator?.locations?.progression, 1.0);
expect(state.currentLocator?.locations?.totalProgression, 1.0);