AVRO-4304: [java][python] Add shared must-reject binary interop vectors

[iemejia]

What is the purpose of the change

Decoder hardening across the SDKs now rejects a range of malformed binary
encodings (overlong varints, Long.MIN_VALUE collection block counts,
out-of-range union branch and enum symbol indices, negative bytes/string
lengths), but there is no shared cross-language fixture guaranteeing that every
SDK rejects the same malformed inputs identically, so the SDKs can drift (one
accepts what another rejects).

This adds a shared set of must-reject vectors under
share/test/data/binary-rejections.json, each a schema plus a raw binary
payload (hex) that a conformant decoder must reject with a bounded,
well-defined error rather than accepting it, crashing, or exhausting memory.
It seeds the set with cases already fixed per SDK, and wires thin per-SDK
harnesses (Java and Python) that load the vectors and assert rejection.

The shared fixture is a single cross-language artifact, so it is kept together
with its first consumers here; additional SDK harnesses can be added later as
small changes on top of the merged fixture.

Verifying this change

This change added tests and can be verified as follows:

• Added share/test/data/binary-rejections.json with 9 seed vectors.
• Added the Java harness TestBinaryDecodingRejections, which asserts each
  vector is rejected on both the classic and fast reader paths.
• Added the Python harness test_binary_decoding_rejections.py, which asserts
  each vector is rejected with a bounded AvroException.

Documentation

• Does this pull request introduce a new feature? (no — shared conformance test
  fixtures)
• If yes, how is the feature documented? (the fixture file carries a
  self-describing description field explaining the format and intent)

[Copilot]

Pull request overview

Adds a shared, cross-language “must-reject” fixture of malformed Avro binary payloads and introduces thin Java/Python test harnesses to assert that decoders reject those payloads with bounded errors (preventing cross-SDK drift in decoder hardening behavior).

Changes:

• Added share/test/data/binary-rejections.json containing seed malformed binary vectors (schema + hex payload + category).
• Added a Java parameterized test harness that loads the shared vectors and asserts both classic and fast-reader paths reject them.
• Added a Python unittest harness that locates the shared vectors in a source checkout and asserts the decoder rejects each payload with AvroException.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
share/test/data/binary-rejections.json	Introduces the shared cross-SDK reject-vector manifest (schema + hex payloads).
lang/py/avro/test/test_binary_decoding_rejections.py	Python harness that loads the shared vectors and asserts bounded decoder rejection.
lang/java/avro/src/test/java/org/apache/avro/TestBinaryDecodingRejections.java	Java harness that loads the shared vectors and asserts rejection on classic + fast reader paths.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (2)

lang/java/avro/src/test/java/org/apache/avro/TestBinaryDecodingRejections.java:114

• Schema.Parser().parse(schemaJson) happens inside decode() which is executed inside assertThrows(Exception.class, ...). This means an invalid fixture schema (SchemaParseException) would still satisfy the assertion and be reported as a “pass”, even though decoding was never exercised. Also, assertThrows(Exception.class, …) accepts any Exception (e.g. NullPointerException), which weakens the “bounded, well-defined Avro/IO error” intent of the harness.

  private static void decode(String schemaJson, byte[] bytes, boolean fastReader) throws IOException {
    Schema schema = new Schema.Parser().parse(schemaJson);
    GenericData data = new GenericData();
    data.setFastReaderEnabled(fastReader);
    GenericDatumReader<Object> reader = new GenericDatumReader<>(schema, schema, data);

lang/py/avro/test/test_binary_decoding_rejections.py:63

• The fixture is read with manifest.read_text() without an explicit encoding, which can break on systems whose default locale encoding is not UTF-8. Also, schema/hex parsing happens before subTest, so a single malformed vector would fail the whole test without identifying the offending vector in the test output.

        manifest = _find_manifest()
        vectors = json.loads(manifest.read_text())["vectors"]
        self.assertTrue(vectors, "no reject vectors found")
        for vector in vectors:
            name = vector["name"]