release-25.4.6-rc: kvserver: create a fresh context per-store-rebalancer loop iteration

[arulajmani]

Backport 1/1 commits from #164202 on behalf of @arulajmani.

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kvserver: create a fresh context per-store-rebalancer loop iteration

We've seen this cause issues in older versions of CRDB, where we don't have fastValueCtxs. There, the depth of the context chain increases once per-iteration, which is unnecessary.

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Release justification: bug fix for context chain depth growth in store rebalancer

[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.

[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 

[blathers-crl]

❌ PR #164361 does not comply with backport policy

Confidence: high
Explanation: This PR is proposed as a backport based on the resolution of increased context chain depth in older versions of the store rebalancer. This change does not clearly match the predefined critical bug criteria as it does not affect stability or security, lead to data loss/corruption, cause significant performance regressions, result in incorrect query results, nor make debugging more difficult in a way that is critical. It appears to be a performance and quality of life improvement but lacks direct and immediate production impact as defined by critical bug criteria.
Recommendation: Reconsider the backport as it does not meet the critical bug fix criteria or ensure it is introduced behind a feature flag to maintain compliance with backport policies.

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

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