feat(xtypes): Add validation function to String type

[FranciscoKurpiel]

The xtypes.String type previously included a ValueValid method that was a no-op, always returning nil. This prevented consumers from implementing custom validation logic for string values.

This commit introduces an optional ValidateFn field to the String struct. This function, if provided, is executed by the ValueValid method to determine if a given string value is valid.

If the ValidateFn is not set, ValueValid returns nil as before, ensuring that existing code continues to function without modification. This enhancement makes the String type more flexible and allows for more robust input validation where specific formats or constraints are required.

[gemini-code-assist]

Summary of Changes

Hello @FranciscoKurpiel, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the xtypes.String type by integrating a mechanism for custom validation. Previously, the string type lacked a functional validation method, limiting its utility for enforcing specific data formats or constraints. The changes introduce an optional validation function that can be supplied by the user, allowing for tailored checks on string values. This makes the String type more adaptable and powerful for various application requirements.

Highlights

• Custom Validation for String Type: Introduced an optional ValidateFn field to the xtypes.String struct, allowing consumers to define and apply custom validation logic for string values.
• Enhanced ValueValid Method: The ValueValid method now executes the provided ValidateFn if it is set. If ValidateFn is not configured, the method retains its previous behavior of returning nil, ensuring backward compatibility.
• Increased Flexibility: This enhancement makes the String type more flexible, enabling more robust input validation where specific formats or constraints are required, moving beyond the previous no-op validation.

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[gemini-code-assist]

Code Review

This pull request introduces a flexible validation mechanism for the xtypes.String type by adding an optional ValidateFn. While this is a valuable addition, the current implementation of ValueValid is not thread-safe and could lead to a panic if the String struct's configuration is modified concurrently. My review includes a high-priority comment with a suggested fix to address this race condition.