Fix redefining a named Avro type in a diamond dependency pattern

[Robert Yokota (rayokota)]

What

Fix redefining a named Avro type in a diamond dependency pattern. Fixes #2236

Checklist

• [Y] Contains customer facing changes? Including API/behavior changes
• [Y] Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required

References

JIRA:

Test & Review

Open questions / Follow-ups

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Pull request overview

Fixes Avro schema reference resolution for “diamond dependency” graphs where multiple sibling references share the same named type, avoiding SchemaParseException("redefined named type ...") during parsing.

Changes:

• Update _resolve_named_schema (sync + async) to store referenced schemas in raw (unparsed) form so the top-level load_schema resolves each named type only once.
• Add sync and async regression tests that reproduce the diamond-dependency reference pattern and verify serialize/deserialize round-trips.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/confluent_kafka/schema_registry/_sync/avro.py	Adjusts named-schema reference collection to prevent duplicate inline definitions in diamond graphs.
src/confluent_kafka/schema_registry/_async/avro.py	Async equivalent of the named-schema reference resolution fix.
tests/schema_registry/_sync/test_avro_serdes.py	Adds regression test covering diamond reference dependencies for sync serializer/deserializer.
tests/schema_registry/_async/test_avro_serdes.py	Adds regression test covering diamond reference dependencies for async serializer/deserializer.

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[sonarqube-confluent]

Quality Gate passed

Issues
2 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
90.9% Coverage on New Code
15.6% Duplication on New Code

See analysis details on SonarQube