Add support for underscores to be in callback assert custom class

[Kamefrede]

This PR adds support for underscores to be considered valid in the callback assert custom class name.

[Copilot]

Pull request overview

Adds support for underscores in CallbackAssert custom class names by relaxing the validation regex, and introduces a regression test intended to verify the new accepted format.

Changes:

• Allow _ in callback assert customClass name validation.
• Add/adjust tests for underscore-containing custom class names.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/asserts/callback-assert.js	Updates the custom class name validation regex to accept underscores.
test/asserts/callback-assert.test.js	Adds tests intended to validate underscore support (currently introduces failing/inconsistent assertions).

Comments suppressed due to low confidence (1)

test/asserts/callback-assert.test.js:111

• This new "should not throw" test duplicates the existing one above and doesn't assert anything (so it would pass even if an exception is thrown by a different async context later). Consider removing it, or converting it to the same assert.doesNotThrow(() => ...) pattern and/or using it to specifically validate that an underscore-containing customClass passes when the callback returns true.

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