Fix Encryption fails when expanded union types have two references to the same record (#2262)

[ChristophMajcenAtXxxlutz]

What

Fix Encryption fails when expanded union types have two references to the same record

Checklist

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

References

Github: #2262

Test & Review

Automatic tests

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[Copilot]

Pull request overview

Note

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Fixes an issue where encryption fails when expanded union types contain two references to the same record. Adds a _collapse_schema helper that re-collapses already-expanded named type references before validation in _resolve_union.

Changes:

• Add _collapse_schema function to collapse expanded named-type references back to name references before validating union members.
• Use _collapse_schema on a deep copy of the subschema in _resolve_union prior to validate().
• Add sync and async tests covering union with two references to the same record type, plus a CHANGELOG entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

File	Description
src/confluent_kafka/schema_registry/common/avro.py	Adds _collapse_schema helper and uses it during union resolution validation.
tests/schema_registry/_sync/test_avro_serdes.py	Adds sync test for encryption with union containing repeated record refs.
tests/schema_registry/_async/test_avro_serdes.py	Adds async equivalent test.
CHANGELOG.md	Documents the fix.

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[Robert Yokota (rayokota)]

/sem-approve

[Robert Yokota (rayokota)]

ChristophMajcenAtXxxlutz , please run make style-fix

[ChristophMajcenAtXxxlutz]

Robert Yokota (@rayokota) Fixed the styling issues

[Robert Yokota (rayokota)]

/sem-approve

[Robert Yokota (rayokota)]

ChristophMajcenAtXxxlutz , looks like there's a mypy error now:

Running mypy type checking ...02:08
src/confluent_kafka/schema_registry/common/avro.py:270: error:�(B Unsupported right operand type for in (�(B"Any | None"�(B) �(B[operator]�(B02:12
Found 1 error in 1 file (checked 106 source files)�(B

[Robert Yokota (rayokota)]

/sem-approve

[Robert Yokota (rayokota)]

Thanks ChristophMajcenAtXxxlutz , LGTM