release-25.2: roachtest: ignore flaky npgsql test

[bghal]

Backport 1/1 commits from #167474 on behalf of @bghal.

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The Npgsql.Tests.CommandTests(NonMultiplexing).Cancel_async_soft has
been timing out.

Epic: none
Fixes: #167069

Release note: None

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[trunk-io]

😎 Merged directly without going through the merge queue, as the queue was empty and the PR was up to date with the target branch - details.

[blathers-crl]

✅ PR #167479 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR changes a test file specifically named 'npgsql_blocklist.go' under the 'pkg/cmd/roachtest' directory, which falls under the non-production file patterns outlined in the backport policy for CockroachDB. Specifically, the 'pkg/cmd/roachtest/' path indicates that the file relates to development tools used for testing, thus exempting it from critical bug justification or feature flag requirements.

❌ ENGREQ Check Failed: Please check one of the backport category checkboxes.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[github-actions]

/trunk merge