release-26.2: opt: add optimizer_span_limit and use in GenerateConstrainedScans

[michae2]

Backport 1/2 commits from #167180.

/cc @cockroachdb/release

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opt: add optimizer_span_limit and use in GenerateConstrainedScans

This commit creates a new session variable optimizer_span_limit, and
uses it to bound the number of spans in constrained scans produced by
GenerateConstrainedScans.

Fixes: #167620

Release note (sql change): Add a new session variable
optimizer_span_limit which bounds the number of spans the optimizer
will allow in a single constrained index scan. If a single IN set has
more items than this limit, that IN set will not be used to build a
constrained index scan. If the cross product of two or more IN sets
would produce more spans than this limit for a composite index, then
only a prefix of the IN sets will be used to produce spans.

For example, for the following table and query, only the predicates on
columns a and b will be used to construct the constrained scan of
abc_idx, because including the predicate on column c would produce
more spans than optimizer_span_limit:

CREATE TABLE abc (a INT, b INT, c INT, INDEX abc_idx (a, b, c));
SET optimizer_span_limit = 10;
SELECT * FROM abc WHERE a IN (1, 3, 5) AND b IN (2, 4, 6) AND c IN (7, 9, 11);

Co-Authored-By: roachdev-claude roachdev-claude-bot@cockroachlabs.com

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Release justification: disabled-by-default fix for customer workload.

[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.

[trunk-io]

😎 Merged successfully - details.

[cockroach-teamcity]

This change is 

[blathers-crl]

Detected infrastructure failure (matched: self-hosted runner lost communication with the server). Automatically rerunning failed jobs. (run link)

[michae2]

Thanks!

/trunk merge