Fix canonicalization resulting in empty list

[RobinMalfait]

This PR fixes a bug in the canonicalization process where if a few utilities collapse into a smaller one, and the smaller one is part of the original list, then it results in an empty list.

It will be more clear with an example. Let's say you have this setup:

w-[calc(1rem+0.25rem)] h-[calc(1rem+0.25rem)] size-5

The first step is that this will result in:

w-5 h-5 size-5

Then the w-5 h-5 can turn into size-5. But the existing size-5, can also be replaced by the size-5.

Internally, when we have a replacement, then we mark all the classes that can be replaced as "droppable", so they would be dropped from the list. But in this scenario we also marked size-5 as droppable, resulting in an empty list.

If an additional class existed:

w-[calc(1rem+0.25rem)] h-[calc(1rem+0.25rem)] size-5 flex

The result would be

flex

Instead of the expected:

size-5 flex

Test plan

1. Existing tests pass
2. Added new tests with and without an additional class

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• CHANGELOG.md

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• CHANGELOG.md

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Walkthrough

The change adds new tests to packages/tailwindcss/src/canonicalize-candidates.test.ts covering duplicate utilities, calc-based width/height values, and interactions with size-* and other classes. The collapseGroup implementation in packages/tailwindcss/src/canonicalize-candidates.ts was modified to add a replacement to results earlier, drop only non-replacement items from combinations, and preserve a replacement if it is already present in the combo. CHANGELOG.md was updated with a fix note about previous canonicalization producing an empty list in some cases.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main bug being fixed: a canonicalization process that was producing empty lists.
Description check	✅ Passed	The description is well-detailed, providing a clear explanation of the bug, concrete examples, and the test plan for validating the fix.

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