Apache decoder fails with `UnsupportedOperationException` when `AvroParser.Feature.AVRO_BUFFERING` disabled

[cowtowncoder]

Problem

When AvroParser.Feature.AVRO_BUFFERING is disabled and the Apache decoder is used
(ApacheAvroFactory), parsing from an InputStream fails immediately with
UnsupportedOperationException: the very first nextToken() call throws, so no
content can be read at all.

Root cause: with buffering disabled, ApacheAvroParserImpl constructs the decoder with
DecoderFactory.directBinaryDecoder(), but RootReader.nextToken() calls
ApacheAvroParserImpl.checkInputEnd(), which delegates to BinaryDecoder.isEnd() --
and Apache's DirectBinaryDecoder.isEnd() is an unconditional
throw new UnsupportedOperationException().

java.lang.UnsupportedOperationException
	at org.apache.avro.io.DirectBinaryDecoder.isEnd(DirectBinaryDecoder.java:194)
	at com.fasterxml.jackson.dataformat.avro.apacheimpl.ApacheAvroParserImpl.checkInputEnd(ApacheAvroParserImpl.java:231)
	at com.fasterxml.jackson.dataformat.avro.deser.RootReader.nextToken(RootReader.java:28)
	at com.fasterxml.jackson.dataformat.avro.deser.AvroParserImpl.nextToken(AvroParserImpl.java:97)

Reproduction

AvroMapper vanilla = new AvroMapper();
AvroSchema schema = vanilla.schemaFrom(
    "{'type':'record','name':'Tiny','fields':[{'name':'x','type':'int'}]}".replace('\'', '"'));
byte[] doc = vanilla.writer(schema).writeValueAsBytes(Collections.singletonMap("x", 42));

AvroFactory f = new ApacheAvroFactory();
f.disable(AvroParser.Feature.AVRO_BUFFERING);
AvroMapper mapper = AvroMapper.builder(f).build();

// works: byte[] input
mapper.readerFor(Map.class).with(schema).readValue(doc);                          // -> {x=42}

// fails: InputStream input
mapper.readerFor(Map.class).with(schema)
    .readValue(new ByteArrayInputStream(doc));                                    // -> UnsupportedOperationException

Scope

Only the combination of all three matters:

• Apache decoder (ApacheAvroFactory), AND
• AVRO_BUFFERING disabled, AND
• InputStream (not byte[]) input

byte[] input is unaffected because that constructor always uses
DECODER_FACTORY.binaryDecoder(buffer, ...) and ignores the feature entirely.
The non-Apache (native JacksonAvroParserImpl) decoder is unaffected.

Note that AvroFactoryBuilder.builderWithApacheDecoder() is currently a no-op
(the flag is copied to AvroFactory but never read by _createParser), so reaching
this requires constructing ApacheAvroFactory directly -- which is probably why it
has gone unnoticed.

Affects

Present since 2.16.0 (323373de, where direct-decoder support was added), and still
present on 2.18, 2.x and 3.x. No existing test parses with this combination:
MapperConfigTest only asserts the feature flag's state, never exercises decoding.

Possible fixes

1. Track end-of-input in ApacheAvroParserImpl instead of delegating to
   BinaryDecoder.isEnd() when a direct decoder is in use; or
2. ignore AVRO_BUFFERING for the Apache decoder (always buffer) and document that; or
3. reject the combination explicitly at parser construction, so failure is a clear
   configuration error rather than an UnsupportedOperationException mid-parse.

Found while reviewing #796.

[Dongnyoung]

@cowtowncoder May I work on this?🧐