[release/9.0] Update the JSON schema exporter to reuse schemas more aggressively.

[github-actions]

Backport of #108764 to release/9.0

/cc @eiriktsarpalis

Customer Impact

• Customer reported
• Found internally

A customer reported that generating a JSON schema for typeof(XElement) results in explosive memory growth due to the immense complexity of its underlying type graph. This is caused by the fact that JsonSchemaExporter uses a very conservative schema reuse policy via the $ref keyword, reserving it only for cases where a cycle has been detected in the type graph. This in turn results in a high degree of duplication in clique-like type graphs contributing to exponential memory consumption.

This PR addresses the issue by applying a more aggressive schema reuse policy: if a schema for a node in the graph has been generated, subsequent occurrences will always produce schemas pointing to the original occurrence regardless of its location. This effectively bounds the size of the generated document to $\mathcal O(n)$ where $n$ denotes the number of .NET members occurring in the type graph.

Regression

• Yes
• No

Testing

Added unit testing validating the impacted use case.

Risk

Moderate. Makes a last-minute change to the schema generation algorithm which carries risk. Changes what JSON schema is being produced under certain circumstances, even though the new outputs are semantically equivalent to the original outputs.

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