schemadiff: schema is immutable through Apply()

[shlomi-noach]

Description

Schema.Apply() returns a new, likely modified, schema. But it must not affect the receiver schema.
This PR:

• Uses query export/import to ensure the two schemas do not share any pointers/maps/slices
• Adds tests to validate schema is unaffected by Apply()

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

[github-actions]

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[shlomi-noach]

Tracking issue: #10203