AVRO-4189: [Java] Use ClassUtils.forName() in FastReaderBuilder for consistency

[iemejia]

I noticed that FastReaderBuilder.findClass() was using data.getClassLoader().loadClass() directly, bypassing ClassUtils.forName() which is used everywhere else in the codebase. This change aligns FastReaderBuilder with the standard class loading path and adds tests for class loading behavior.

What is the purpose of the change

(For example: This pull request improves file read performance by buffering data, fixing AVRO-XXXX.)

Verifying this change

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Extended interop tests to verify consistent valid schema names between SDKs
• Added test that validates that Java throws an AvroRuntimeException on invalid binary data
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Documentation

• Does this pull request introduce a new feature? (yes / no)
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