release-26.1: sql: fix nil LeafTxnInputState assertion in inner plans

[blathers-crl]

Backport 1/1 commits from #161222 on behalf of @michae2.

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Previously, when inner plans (such as apply-join iterations or routines) were executed with a LeafTxn inherited from an outer plan, an assertion failure could occur. The issue was that MakeLeafTxn was always set on the flow context, even when LeafTxnInputState was not available.

When a JoinReader in the inner plan called UseStreamer(), it would see that flowCtx.Txn.Type() == kv.LeafTxn and flowCtx.MakeLeafTxn != nil, and attempt to create a new leaf transaction. This would hit the assertion "nil LeafTxnInputState when trying to create the LeafTxn" because inner plans that inherit a LeafTxn cannot call GetLeafTxnInputState() (that method only works on RootTxn).

This commit fixes the issue by only setting MakeLeafTxn on the flow context when LeafTxnInputState is actually available. This way, UseStreamer() will see MakeLeafTxn == nil and simply use the existing flowCtx.Txn directly instead of trying to create a new leaf transaction.

(No release note because this is a rare failure.)

Fixes: #161168

Release note: None

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Release justification: low-risk change to fix a test failure.

[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 

[yuzefovich]

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