feat: Add playback speed control to chapter player

[Sai-Adithya-M]

Description

This PR introduces a Playback Speed Control feature to the Chapter Player. Users can now adjust the audio playback rate to suit their listening preferences.

Key Changes:

• UI: Added a toggle button in
  ChapterPlayer to cycle through speeds: 0.5x, 0.75x, 1.0x, 1.25x, 1.5x, 2.0x.
• Logic: Updated ChapterPlayerController to manage playback speed state and interface with AudioPlayer.
• Tests: Added unit tests to verify speed control logic

Fixes #707

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

I have performed the following tests to verify these changes:

1.Unit Testing: Created and ran
test/controllers/chapter_player_controller_speed_test.dart to verify that
setPlaybackSpeed correctly updates the observable state and calls the audio player API.

• Command: flutter test test/controllers/chapter_player_controller_speed_test.dart
  2.Manual Verification:
• Verified that the speed button appears in the Chapter Player UI.
• Verified that tapping the button updates the label (e.g., 1.0x -> 1.25x) and cycles through all speeds.
• Verified that the audio pitch/speed changes accordingly during playback.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes feat: Add Playback Speed Control to Chapter Player #707
• Tag the PR with the appropriate labels

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