feat: add a __str__ method to CustomFractionalFee class

[MonaaEid]

This pull request:
Enhances the usability of the CustomFractionalFee class by adding a string representation method and updates the related example to use this new method. This makes it easier to inspect and debug instances of CustomFractionalFee.

• Added a __str__ method to the CustomFractionalFee class to provide a clear, formatted string representation of its attributes, improving readability and debugging.

• Updated the custom_fractional_fee.py example to print the string representation of a CustomFractionalFee instance using the new __str__ method.

Related issue(s):

Fixes #916

Checklist

• Add __str__ method to the CustomFractionalFee class
• Updated the custom_fractional_fee.py example
• Update CHANGELOG.md
• Documented (Code comments, README, etc.)
• Tested

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[Copilot]

Pull request overview

This pull request adds a __str__ method to the CustomFractionalFee class to improve debugging and inspection capabilities. The change allows developers to easily view formatted string representations of CustomFractionalFee instances.

• Added a __str__ method to CustomFractionalFee class for string representation
• Updated the example script to demonstrate the new method
• Updated CHANGELOG.md to document the change

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/hiero_sdk_python/tokens/custom_fractional_fee.py	Implements the __str__ method to provide a formatted string representation of the class attributes
examples/tokens/custom_fractional_fee.py	Simplified the example by using the new __str__ method instead of manually printing each attribute
CHANGELOG.md	Added changelog entry documenting the new feature

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