release-25.2: opt: estimate worst-case selectivity of placeholder equalities

[mgartner]

Backport 2/2 commits from #156791.

/cc @cockroachdb/release

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Backport:

• 1/1 commits from "opt: estimate worst-case selectivity of placeholder equalities" (opt: estimate worst-case selectivity of placeholder equalities #151409)
• 1/1 commits from "sql: add optimizer_use_max_frequency_selectivity session setting" (sql: add optimizer_use_max_frequency_selectivity session setting #156654)

Please see individual PRs for details.

/cc @cockroachdb/release

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Release justification: Low-risk planning improvement for generic query plans.

[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.

[cockroach-teamcity]

This change is 

[blathers-crl]

❌ PR #156797 does not comply with backport policy

Confidence: medium
Explanation: The PR intends to make modifications in two areas: enhancing selectivity estimations for query plans and introducing a new session setting named 'optimizer_use_max_frequency_selectivity'. None of these changes appear to address critical bugs as defined by stability, data corruption, or significant performance regressions. The new session setting could be viewed as a feature flag, but the PR lacks explicit evidence that this setting is disabled by default. Additionally, the 'Release justification' line in the PR body mentions a 'low-risk planning improvement' which does not suggest an immediate and critical fix or any sort of gating behind a feature flag necessary for less critical changes, as per the Backport Policy.
Recommendation: Request clarification or addition of details regarding the default state of the new session setting to confirm it as a feature flag. Alternatively, reconsider the backport due to the lack of critical bug fix criteria met.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[michae2]

@michae2 reviewed 17 of 17 files at r1, 10 of 10 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)