Improve toString() for Profiles.of() implementation

[sbrannen]

The toString() implementation in ProfilesParser (which is the internal implementation of Profiles.of()) currently concatenates profile expressions with " or ".

For example, the current string representation of Profiles.of("spring & framework", "java | kotlin") is "spring & framework or java | kotlin".

Ideally, the string representation would be "(spring & framework) | (java | kotlin)".

This makes it easier to read (for example, when debugging) and aligns with the semantics of toStringAST() in SpelNode.

As an additional benefit, the result of invoking toString() could even be used as a logically equivalent composite profile expression, though that is not the primary goal of the proposed change.