Add additional test for STJ Schema deprecated property

[Youssef1313]

Only adds additional test coverage. It occurred to me that I could have got this scenario wrong when I added deprecated implementation in #130665, but turned out to be correct already. So I thought it's worth adding a test.

Basically, we want to ensure that for the following case, we don't end up re-using the same $ref for the two properties. Only Deprecated should get the deprecated property.

public sealed class PocoWithObsoletePropertiesSharingType
{
    public PocoSharedByObsoleteMembers? NotDeprecated { get; set; }

    [Obsolete]
    public PocoSharedByObsoleteMembers? Deprecated { get; set; }
}

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[Copilot]

Pull request overview

Adds targeted System.Text.Json JsonSchemaExporter test coverage to ensure the deprecated keyword is applied only to obsolete members in scenarios where schema nodes/types could otherwise be shared (including $ref reuse paths).

Changes:

• Adds a new test case covering two properties sharing the same POCO type, where only the obsolete property’s schema should be marked deprecated.
• Adds a new test case covering two collection properties sharing a $ref’d element schema, ensuring deprecated does not leak into the shared referenced schema.
• Introduces small POCO helper types used by the new test cases.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.