Use Copy Constructor for JsonSerializerSettings

[MatthewSteeples]

Fixes #29532

Fixes an issue where JsonSerializerSettings.DateFormatString overwrites DateFormatHandling. See #29532 for details

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[MatthewSteeples]

@pranavkm the bot hasn't worked out who needs to review this and you were involved in the linked issue. Any chance you could nudge it in the right direction?

[halter73]

Thanks for the contribution!

[MatthewSteeples]

I can't work out where the latest lot of build failures are coming from, so I'm hoping it's not me...!

[MatthewSteeples]

@halter73 I still can't work out why the test is failing but I'm relatively sure it's nothing I'm doing. Do you have any pointers for how to proceed from here?

[Copilot]

Pull request overview

Updates MVC’s Newtonsoft.Json output formatter to clone JsonSerializerSettings via Json.NET’s copy constructor, avoiding the DateFormatString setter side-effect that can cause DateFormatHandling to be ignored (per #29532).

Changes:

• Replace the formatter’s custom ShallowCopy logic with new JsonSerializerSettings(SerializerSettings).
• Remove the now-unneeded ShallowCopy helper.
• Add a regression test covering the new copy-constructor-based behavior (but see comment about strengthening the assertion).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Mvc/Mvc.NewtonsoftJson/src/NewtonsoftJsonOutputFormatter.cs	Switches settings cloning to Json.NET’s copy constructor and removes ShallowCopy.
src/Mvc/Mvc.NewtonsoftJson/test/NewtonsoftJsonOutputFormatterTest.cs	Adds a test intended to validate that the copy constructor doesn’t override date formatting behavior.

[Copilot]

This test only compares DateFormatHandling/DateFormatString property values. The original bug is caused by a hidden side effect of setting DateFormatString (even to null) which flips an internal "DateFormatString has been set" flag; property equality will still pass even when the bug is present. Please assert the observable behavior instead (e.g., serialize a DateTime using the copied settings or via NewtonsoftJsonOutputFormatter and verify the payload uses MicrosoftDateFormat rather than the default ISO date string).

Suggested change

	// Act
	var copiedSettings = new JsonSerializerSettings(originalSettings);
	// Assert
	Assert.Equal(originalSettings.DateFormatHandling, copiedSettings.DateFormatHandling);
	Assert.Equal(originalSettings.DateFormatString, copiedSettings.DateFormatString);
	var value = DateTime.UnixEpoch;
	// Act
	var copiedSettings = new JsonSerializerSettings(originalSettings);
	var originalJson = JsonConvert.SerializeObject(value, originalSettings);
	var copiedJson = JsonConvert.SerializeObject(value, copiedSettings);
	// Assert
	Assert.Equal(originalJson, copiedJson);
	Assert.Equal(@"""\/Date(0)\/""", copiedJson);

[BrennanConroy]

/ba-g unrelated hang