feat(model-avro): re-platform converter onto common-avro + common-json

[jfallows]

Fixes #1856.

Re-platforms the model-avro converter off the shaded Apache Avro + Jackson whole-message GenericRecord/JSON path onto the format-native common-avro library (#1838/#1884) for the Avro side and the common-json transcoder (#1855) for the JSON side, surfaced through the engine model SPI.

common-avro — canonical union mode

Adds a canonical (flattened nullable-single union) mode to the AvroJson bridge:

• AvroJson.parser/stream/generator(schema, ..., canonical) — when canonical is true, a union of null and exactly one other type encodes/decodes as a bare JSON value (null or the unwrapped value) instead of the {"<branch>": value} wrapper. Every other union shape is unchanged.
• This is what lets model-avro's view: json keep its existing key=value semantics for ["null", T] unions, matching the prior CanonicalJsonEncoder.
• Covered by new AvroJsonTest cases.

model-avro — re-platform

• AvroRead/AvroWrite converters drive the common-avro AvroJson streaming pipelines (avro↔JSON, canonical) for view: json; the schema is resolved via CatalogHandler, compiled to an AvroSchema, and cached per schemaId alongside its pull parser.
• Validation and $.field extraction run on the common-avro streaming pull parser (zero-copy segments for string/bytes/fixed); a decode/validation failure aborts via engine RESET.
• Drops the shaded Apache Avro + Jackson dependencies, the shade plugin, and the org.apache.avro.io.* classes; depends on common-avro + common-json; NOTICE regenerated.
• Preserves existing model: avro config, view: json semantics, JSON padding, and field extraction. The validation-failed event now carries the common-avro decode error, mirroring the model-json re-platform precedent (EventIT + event.yaml updated accordingly).

Testing

Full clean install with k3po integration tests is green for model-avro.spec, common-avro, and model-avro: unit tests, EventIT, JaCoCo coverage, Checkstyle, license, and NOTICE checks all pass.

Notes

• Verified build was scoped to the affected modules. The public API (io.aklivity.zilla.runtime.model.avro.config) is unchanged, so dependents (docker image, bindings using the avro model) are not expected to be affected.
• The spec's expected validation event now includes the literal truncated datum error string from common-avro; if that wording changes upstream the spec expectation needs a matching update — the same coupling model-json already has.

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