release-25.4: rowenc: fix panic in composite datum type checking

[blathers-crl]

Backport 1/1 commits from #161739 on behalf of @ZhouXing19.

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Fixes #161402

Previously, composite type checking assumed all values implemented tree.CompositeDatum, causing panics when this wasn't true. Use safe type assertion to check if datum is composite before calling IsComposite().

Release note: None

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Release justification: fix for high severity stability issue that causes node crashes. A test failure revealed that incorrect type assumptions in rowenc can trigger panics when processing composite types, leading to node instability and potential service disruption.

[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. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. 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.

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.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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

[blathers-crl]

✅ PR #161746 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Stability or security issues]
Backward compatible: true
Explanation: The pull request is compliant with the backport policy as it addresses a critical bug and includes a clear release justification description. The changes are made to a production file ('pkg/sql/rowenc/index_encoding.go') to address a significant stability issue in handling composite types, which was causing node crashes. This correction qualifies as fixing a stability issue under the backport policy's critical bug criteria. It uses a safer type assertion method to avoid panics when the datum is not a composite type. Such a fix directly improves the stability of the CockroachDB nodes and prevents service disruption, a compelling reason for a backport without the need for feature flags. The change is backward compatible as it does not introduce or remove existing functionality or version gates; instead, it refines the existing functionality by preventing unhandled conditions that lead to failures.

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

[cockroach-teamcity]

This change is 

[michae2]

@michae2 reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich and @ZhouXing19).

[ZhouXing19]

/trunk merge

[trunk-io]

😎 Merged successfully - details.