Fix PA_PUBLIC_PRIMITIVE_ATTRIBUT of MuttablePair class with attribute encapsulation

[felipevendrami]

Encapsulate fields left and right in MutablePair to improve data integrity

Description:
This pull request addresses the issue identified by SpotBugs (PA_PUBLIC_PRIMITIVE_ATTRIBUTE) regarding the "left" and "right" fields in the MutablePair class. These fields were publicly accessible, which exposed them to potential unintended modification, violating the principle of encapsulation.

Changes made:

1. Encapsulation: The fields "left" and "right" in MutablePair were changed from public to private to improve data integrity.
2. Getter methods: The getLeft() and getRight() methods were already present, so no additional getters were created.
3. Test class update: Updated MutablePairTest to use the getLeft() and getRight() methods instead of accessing the fields directly.

SpotBugs Reference:
This change resolves the SpotBugs warning: PA_PUBLIC_PRIMITIVE_ATTRIBUTE
Reference: MutablePair.java:[line 138]

Why this change is important:
Encapsulating fields ensures better control over the internal state of the MutablePair object, preventing potential misuse and maintaining consistency within the class.

[garydgregory]

-1: This PR breaks binary compatibility.

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