release-25.2: 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.

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[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?

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[blathers-crl]

✅ PR #161744 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 CockroachDB backport policy as it addresses a high-severity stability issue which constitutes a critical bug under the policy specifics. The description and the attached labels corroborate that the change is intended to fix an issue causing node crashes, thereby directly contributing to maintaining or restoring stability to the node operation. This meets the critical bug criteria, specifically improving stability issues and preventing node crashes. Moreover, the change has a release justification included in the PR body which explicitly states that this is a necessary fix for a stability issue.

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

[cockroach-teamcity]

This change is 

[michae2]

LGTM

[github-actions]

Reminder: it has been 2 weeks please merge or close your backport!