release-25.2: sql/stats: handle range counts when skipping dropped enum hist buckets

[michae2]

Backport 1/2 commits from #155035.

/cc @cockroachdb/release

---

opt/props: only check histogram NumRange=0 in test builds

These assertions that the first histogram bucket has NumRange=0 are
useful for catching bugs, but a liability in production
environments. Even with malformed histograms, we should not be failing
query execution. Only check these assertions in test builds.

Informs: #154461

Release note (bug fix): Change assertions about histogram NumRange=0 to
only be checked in test builds.

---

Release justification: low-risk fix for a critical support issue.

[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.}

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #155651 is compliant with backport policy

Confidence: high
Critical bug criteria met: [Stability or security issues Bugs that can cause the DB to return incorrect results or result in suboptimal performance]
Backward compatible: true
Explanation: The PR introduces a modification which ensures that certain assertions related to the 'NumRange' of histogram buckets are only checked in test builds, thus not affecting production systems. The primary change is in making this check available only in a test environment to avoid query failures due to malformed histograms in production. This aligns with improving system stability and avoiding unnecessary failures, making it a critical bug fix. The changes in the PR do not appear to require a new feature flag because they are specifically targeted at coding changes that prevent potential disruptions in production. While there is the addition of a test-only check, it does not alter the production functionality or introduce backward-incompatible changes. This change makes the system more robust without altering its expected functional behaviors outside of testing scenarios.

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

[yuzefovich]

@yuzefovich reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @michae2)