Add ValueComparer for JSON DOM types using DeepEquals

[hostage2222]

The change tracker currently uses shallow comparison when detecting changes to JSON DOM properties (JsonDocument and JsonElement). If the JSON content itself hasn't actually changed, but a new object instance was created, this results in unnecessary saves to the database.

This PR adds ValueComparers for JsonDocument and JsonElement based on DeepEquals.

Trade-offs:

• For performance, the snapshot stores the original JsonDocument/JsonElement instance rather than a deep copy. If the original JsonDocument has since been disposed, comparing against the snapshot will throw.
• Key order doesn't matter for jsonb, but does for json. DeepEquals follows jsonb's semantics either way.
• JsonElement.GetHashCode() is inefficient, so JsonElementComparer always returns 0 as its hash.
• JsonDocumentComparer's GetHashCode() is reference-based while Equals compares by content, breaking the Equals/GetHashCode contract — same trade-off as HstoreMutableComparer for Dictionary<string, string>.

As a very rough estimate of the potential performance gain, I benchmarked saving 1 million JsonDocuments to the database, recreating the objects each time without actually changing their content:

• ~24 sec with the old comparer (unconditional save)
• ~0,7 sec with the new comparer (save skipped — no actual change detected)

Relates to #3314

[Copilot]

Pull request overview

This PR improves EFCore.PG change tracking for JSON DOM-mapped properties (JsonDocument, JsonElement) by introducing provider-specific ValueComparers that use JsonElement.DeepEquals, reducing unnecessary UPDATEs when JSON content is unchanged but object instances differ.

Changes:

• Add ValueComparer implementations for JsonDocument and JsonElement to NpgsqlJsonTypeMapping, using deep JSON equality semantics.
• Extend unit tests to validate deep equality behavior for the JSON DOM comparers.
• Add functional tests asserting that SaveChanges emits UPDATEs only when JSON content changes (and does not throw for undefined JsonElement cases).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/EFCore.PG/Storage/Internal/Mapping/NpgsqlJsonTypeMapping.cs	Adds JSON DOM ValueComparers and wires them into the JSON type mapping so change tracking compares by JSON content.
test/EFCore.PG.Tests/Storage/NpgsqlTypeMappingTest.cs	Expands comparer unit tests to verify deep equality across new instances, ordering/number formatting cases, and null/undefined scenarios.
test/EFCore.PG.FunctionalTests/Query/JsonDomChangeTrackingTest.cs	New functional coverage validating UPDATE/no-UPDATE behavior for json/jsonb DOM properties and ensuring undefined JsonElement doesn’t throw during DetectChanges.

[hostage2222]

This isn't actually an issue. ValueComparer has the following constructor:

public ValueComparer(
Expression<Func<T?, T?, bool>> equalsExpression,
Expression<Func<T, int>> hashCodeExpression,
Expression<Func<T, T>> snapshotExpression)
: base(equalsExpression, hashCodeExpression, snapshotExpression)
{
}

Note that unlike equalsExpression, hashCodeExpression is typed as Func<T, int> rather than Func<T?, int> — T isn't nullable here, so this is expected.