release-25.2: sql: fix early exit from merge index planner

[blathers-crl]

Backport 1/1 commits from #155261 on behalf of @mw5h.

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The MergeIndexes() method of the IndexBackfillerMergePlanner has a check to see if there are any spans to merge before proceeding with building the plan for the merge. Previously the check for this was checking whether there were any elements in spansToDo, which is allocated to have one entry per source index ID, causing the length check to be vacuously true. The logic probably meant to to check the length of the spans list for each individual source index ID, which this patch puts into place.

Epic: none
Release note: None

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Release justification:

[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?

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[blathers-crl]

❌ PR #155267 does not comply with backport policy

Confidence: high
Explanation: The pull request does not meet the CockroachDB Backport Policy requirements. The changes are made in a production file, which is pkg/sql/mvcc_backfiller.go, and there is no evidence of gating behind a feature flag. Additionally, the PR does not contain a valid 'Release justification' in the body, nor does it fix a critical bug. The described changes address a bug related to the functionality of merging index plans, which does not seem to directly impact stability, security, data loss, significant performance regressions, or produce incorrect results/suboptimal performance beyond the intended functionality. Therefore, it does not qualify under the critical bug criteria explicitly. The PR does not alter any version gates nor includes backward-incompatible changes, so it is backward compatible. However, due to lack of a valid release justification and absence of a feature flag, it is non-compliant.
Recommendation: Reconsider the backport or provide a valid release justification and ensure compliance with feature flag requirements.

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[cockroach-teamcity]

This change is