fix constraint diffing

[morgo]

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The auto-named constraints generate random string names. The fix is to remove them from compares and look at the constraint definition specifically.

[Copilot]

Pull request overview

Improves schema diffing for CREATE TABLE constraints by avoiding spurious diffs when CHECK constraints are logically equivalent but end up with different MySQL auto-generated names due to expression normalization differences (e.g., charset introducers).

Changes:

• Add constraint matching logic in diffConstraints to pair equivalent constraints across differing names and skip DROP/ADD.
• Refactor constraint equality to reuse a new constraintsEqualIgnoreName helper.
• Add regression tests covering charset-introducer normalization and differing-name/differing-expression behavior.

Reviewed changes

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

File	Description
pkg/statement/diff.go	Adds expression-equivalent constraint pairing across names and introduces constraintsEqualIgnoreName.
pkg/statement/diff_test.go	Adds tests to ensure no diff for normalized CHECK expressions with different names, and still diff when expressions differ.

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