release-25.3: 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 the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[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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[cockroach-teamcity]

This change is 

[michae2]

LGTM. I believe v25.3.8 is the last scheduled release of 25.3, so if you feel strongly that this needs to be in 25.3 you should also backport to release-25.3.8-rc. (But it's also ok to just backport to this branch in case we end up cutting another patch release.)

[github-actions]

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