minor: fix issue that fails datasketches tests if they run in certain order(s)

[capistrant]

Using the shared object mapper after #19881 creates a strict test ordering requirement before this proposed patch. Running DatasketchesProjectionTest after some test classes results in failure. switching to private local object mapper removes the ordering dependency

[FrankChen021]

I have reviewed the code for correctness, edge cases, concurrency, and integration risks; no issues found.

Reviewed 1 of 1 changed files.

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This is an automated review by Codex GPT-5.6-Luna(max)

[FrankChen021]

I checked all nine parameterized Datasketches test classes migrated in #19881. DatasketchesProjectionTest is the only one using the shared TestHelper.JSON_MAPPER for module registration together with the default IndexBuilder.create(). I found no other test cases with the same order-dependent AggregatorFactory deserialization hazard.

Although JUnit 5’s @ParameterizedClass could restore class-level parameterization, it is still an experimental API and would not eliminate the shared global mapper state. (JUnit API)