[release/9.0] Make GetPropertyCount public and fix its return value

[github-actions]

Backport of #106503 to release/9.0

/cc @eiriktsarpalis @etemi

Customer Impact

• Customer reported
• Found internally

The implementation of the new JsonElement.DeepEquals in .NET 9 makes use of the internal JsonElement.GetPropertyCount() to short-circuit equality checks for two JSON objects. In a subsequent PR for .NET 10 that makes the GetPropertyCount() method public it was discovered that the method reports inaccurate information (the number of total tokens contained within an object as opposed to the number of properties).

While this inaccuracy doesn't impact functional correctness for equality comparison (if two JSON objects are equal, they always contain an equal number of tokens) it does impact performance, as it can force recursive equality checks for objects containing different numbers of properties.

It should be noted that this change adds new API (JsonElement.GetPropertyCount()) which makes testing for the correct behavior possible.

Regression

• Yes
• No

Testing

Added unit testing validating that the correct property counts are being returned.

Risk

Low. Fixes a straightforward omission in the calculation of JsonElement object sizes that was not being used until .NET 9.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[carlossanlop]

All release/9.0 PRs are currently only requiring M2 approval. @artl93 (or maybe @jeffschwMSFT at the moment)?

[jeffschwMSFT]

approved. please get a code review. this can be merged when ready

[jeffhandley]

Looks good and is approved for 9.0 RC2 once CI is green.

[carlossanlop]

All requirements met. Merging now.