Update (usage of) PyROS UncertaintySet validation routines

[shermanjasonaf]

Summary/Motivation:

Following up on PRs #3558 and #3877, this PR modifies the (usage of) PyROS uncertainty set validation methods.

Changes proposed in this PR:

• Remove UncertaintySet.is_nonempty() call from UncertaintySet.validate(). The call to UncertaintySet.is_nonempty() is considered redundant in the context of PyROS.solve(), since PyROS already checks, outside of UncertaintySet.validate(), the stricter criterion that the uncertainty set contains the user-provided nominal point.
• Ensure that UncertaintySet.is_nonempty() always returns True if the uncertainty set is nonempty, even if the uncertainty set does not contain the user-provided nominal point (see Update PyROS Uncertainty Set Validation Methods #3558).
• Fix the is_nonempty() and is_bounded() methods for DiscreteScenarioSet.
• Update/extend documentation and testing of UncertaintySet.is_bounded(), UncertaintySet.is_nonempty(), UncertaintySet.validate(), and UncertaintySet subclass instance validation routines.
• Simplify UncertaintySet base class implementation.

AI-Use Disclosure

• AI tools were NOT used during the preparation of this PR

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[shermanjasonaf]

@jas-yao @natalieisenberg @jsiirola