release-24.3: opt/execbuilder: pick correct canary column ordinal

[mgartner]

Backport 1/2 commits from #141728.

/cc @cockroachdb/release

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opt/execbuilder: pick correct canary column ordinal

The execbuilder previously used the colOrdMap of an Upsert's input to
find the ordinal of the canary column. In rare cases, the canary column
could also be used as a partial index DEL column. Because a colOrdMap
can only map each column ID to a single ordinal, and because partial
index columns are positioned after the canary column, the ordinal
fetched from the colOrdMap would be the partial index DEL column
ordinal, not the canary column ordinal.

This could cause node-crashing panics during execution when trying to
access the canary canary column in the input row after the partial index
columns are sliced off.

As an example, consider the table and mutation:

CREATE TABLE t (c BOOL PRIMARY KEY, INDEX (c) WHERE c)

UPSERT INTO t VALUES (false)

The input to the Upsert operator produces 4 physical columns:

1. An insert column with the value to insert if a row with the same
   PK does not already exist. We'll call this logical column new_c.
2. A canary column which is non-NULL if a row with the same PK already
   exists. This is always a PK column fetched from the table—in our
   example this is c. Let's call this logical column old_c.
3. A partial index PUT column which is true if the new row should be
   added to the partial index. Since the partial index predicate
   expression is the PK column, this is the same logical column as
   new_c.
4. A partial index DEL column which is true if the old row should be
   removed from the partial index. This is the same logical column as
   old_c.

So, the physical columns produced by the input of the Upsert are:

insert column	canary column	partial index PUT	partial index DEL
new_c	old_c	new_c	old_c

Using the colOrdMap to find the ordinal of old_c yields 3, instead
of 1.

During execution the partial index columns are sliced off, and the row
becomes:

insert column	canary column
new_c	old_c

When the execution engine attempts to access index 3 of this row, the
process panics with an out-of-bounds exception.

This has been fixed by using the lengths of the insert (and other)
columns to find the canary column ordinal, instead of the colOrdMap.

Fixes #136458

Release note (bug fix): A bug has been fixed that could cause gateway
nodes to panic when performing an UPSERT on a table with a BOOL
primary key column and a partial index with the PK column as the
predicate expression.

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Release justification: Low-risk bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[DrewKimball]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @mw5h)