Add grpc connection closing error to allowlist

[sanki92]

Fix TestChangefeedFlushesSinkToReleaseMemory test flake

Fixes #156078

The test was failing on grpc connection closing errors that weren't being ignored. Using grpcutil.IsClosedConnection() to properly handle all connection closing scenarios including the specific "grpc: the client connection is closing" error.

Changed:

• pkg/ccl/changefeedccl/helpers_test.go: Use grpcutil.IsClosedConnection() in error handling

[blathers-crl]

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[cockroach-teamcity]

This change is 

[blathers-crl]

Thank you for updating your pull request.

Before a member of our team reviews your PR, I have some potential action items for you:

• Please ensure your git commit message contains a release note.
• When CI has completed, please ensure no errors have appeared.

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[andyyang890]

Thanks for your contribution! I updated the PR to included the necessary generated code changes and I'll merge it after CI is green.

[andyyang890]

bors r+

[craig]

Build failed:

• unit_tests

[andyyang890]

bors retry

[craig]

Build succeeded:

• linux_amd64_build
• local_roachtest_fips
• lint
• docker_image_amd64
• examples_orms
• check_generated_code
• linux_amd64_fips_build
• local_roachtest
• unit_tests
• acceptance

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #156078: branch-release-25.4.

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