Add negative int32 test coverage for Avro wire format and type widening

[leiykpt]

Summary

This PR adds explicit test coverage proving that negative int32 values are correctly handled at the Avro serialization layer — both binary and JSON wire formats — and that int-to-long type widening preserves sign.

Negative int32 values and very large positive values share overlapping bit representations in two's complement. These tests build confidence that the serialization, deserialization, and schema evolution paths behave correctly for the full signed int32 range.

What's Changed

Added 3 tests to AvroWireFormatCompatibilityTest.java, each exercising three negative values (-1, -100, Integer.MIN_VALUE):

Test	Description
testNegativeInt32BinaryRoundtrip	Serialize negative int to Avro binary, deserialize, verify value preserved
testNegativeInt32JsonRoundtrip	Serialize negative int to Avro JSON, deserialize via compatible decoder, verify value preserved
testNegativeInt32TypeWideningToLong	Write with union[null, int] schema, read with union[null, long] schema, verify sign preserved

Total: 9 new test cases (3 tests x 3 values)

These tests run across all supported Avro versions (1.4 through 1.11) via the existing multi-version test harness. No new schemas or dependencies — tests reuse existing RecordWithNumericFields.avsc and WidenIntToLongInUnionField_{writer,reader}.avsc.

Test Plan

• All 9 new test cases pass locally
• Existing tests in AvroWireFormatCompatibilityTest.java remain green
• CI passes

[dg-builder]

nit but int32 is protobuf/c terminology. We probably want to say int (Avro) or Java integer (Java).