[FLINK-36139][core] Do not use result type hints for aggregate accumulators

[Sunnypain]

What is the purpose of the change

Fix FLINK-36139. For an AggregateFunction<String, Long, Row> implementing ResultTypeQueryable<Row>, accumulator type extraction currently returns the declared Row result type instead of Long. This selects an incompatible serializer for the accumulator.

This is a draft while Jira assignment and maintainer agreement are pending.

Brief change log

• Ignore ResultTypeQueryable only when extracting the AggregateFunction accumulator type; retain the existing generic extraction and input inference.
• Add five regression tests covering concrete and inferred accumulator types, explicit result types, and both missing-type handling modes.

Verifying this change

• Before the fix, the five new tests produced four failures and one pass.
• After the fix, all 80 tests in TypeExtractorTest passed.
• ./mvnw -B -ntp -pl flink-core -am -Dflink.forkCountUnitTest=1 clean verify passed on Temurin 17.0.20.1 with Maven Wrapper 3.9.16. The local run used a separate Maven settings file prioritizing Maven Central. Across the selected modules, JUnit XML reports contain 8,321 tests, zero failures/errors, and 1,777 skipped cases.

The original Kubernetes resubmission/checkpoint scenario and the full Flink repository test suite were not run locally.

Does this pull request potentially affect one of the following parts:

• Dependencies: no.
• Public API: fixes behavior of an existing @PublicEvolving type-extraction method; no signatures or annotations change.
• Serializers: corrects accumulator serializer selection through type information; no serializer implementation or format changes.
• Runtime per-record code paths: no.
• Deployment or recovery: prevents the reported accumulator type mismatch; checkpoint/recovery protocols are unchanged.
• S3 file system connector: no.

Documentation

• Does this pull request introduce a new feature? No.
• Feature documentation: not applicable.

Was generative AI tooling used to co-author this PR?

• Yes.

Generated-by: OpenAI Codex (GPT-6)

[flinkbot]

CI report:

• ed7a83c Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build