Fix Cascades planner treating IS NOT DISTINCT FROM as unusable for index scans

[pengpeng-lu]

Summary

• ScanComparisons.getComparisonType and RangeConstraints classified NOT_DISTINCT_FROM as ComparisonType.NONE instead of EQUALITY, so it could never be folded into an index scan's bounds — it always fell back to a residual FILTER on top of a full/covering scan.
• when the NOT_DISTINCT_FROM comparison sits on the leading key column of
  an index whose ordering doesn't otherwise match the query's ORDER BY, the planner has no way to resolve that
  leading column to a bound value — and the "just scan everything" fallback's natural order doesn't satisfy the
  requested order either. With no viable candidate plan, the Cascades planner throws UnableToPlanException
  ("Cascades planner could not plan query").
• Fix: treat NOT_DISTINCT_FROM as ComparisonType.EQUALITY in ScanComparisons, and add it to RangeConstraints's
  equality-range handling (Range.singleton(...))
• ComparisonTypes. IS_DISTINCT_FROM (its negation) is explicitly left classified as NONE — like NOT_EQUALS, it can't be expressed as a contiguous range, so it correctly remains a residual filter.

Test

• scoped-keyset-pagination.yamsql: end-to-end regression test reproducing the actual UnableToPlanException scenario — a keyset-pagination OR-range on trailing index columns combined with a
  genuinely unbound IS NOT DISTINCT FROM parameter on the index's leading key column. fails with UnableToPlanException pre-fix, passes with the correct union-of-covering-scans plan post-fix.

[normen662]

Can you shorten this AI please?

[normen662]

The PRB seems to be failing

[github-actions]

📊 Metrics Diff Analysis Report

Summary

• New queries: 1
• Dropped queries: 0
• Plan changed + metrics changed: 5
• Plan unchanged + metrics changed: 0

ℹ️ About this analysis

This automated analysis compares query planner metrics between the base branch and this PR. It categorizes changes into:

• New queries: Queries added in this PR
• Dropped queries: Queries removed in this PR. These should be reviewed to ensure we are not losing coverage.
• Plan changed + metrics changed: The query plan has changed along with planner metrics.
• Metrics only changed: Same plan but different metrics

The last category in particular may indicate planner regressions that should be investigated.

New Queries

Count of new queries by file:

• yaml-tests/src/test/resources/scoped-keyset-pagination.metrics.yaml: 1

Plan and Metrics Changed

These queries experienced both plan and metrics changes. This generally indicates that there was some planner change
that means the planning for this query may be substantially different. Some amount of query plan metrics change is expected,
but the reviewer should still validate that these changes are not excessive.

Total: 5 queries

Statistical Summary (Plan and Metrics Changed)

task_count:

• Average change: -8.2
• Median change: -3
• Standard deviation: 10.4
• Range: -29 to -3
• Queries changed: 5
• No regressions! 🎉

task_count % change distribution (5 queries, bin = 2%):

     Range                                  n
----------  ------------------------------  ---
[-4%, -2%)  ████████                        1
[-2%, +0%)  ██████████████████████████████  4

transform_count:

• Average change: -3.4
• Median change: -1
• Standard deviation: 4.8
• Range: -13 to -1
• Queries changed: 5
• No regressions! 🎉

transform_count % change distribution (5 queries, bin = 2%):

     Range                                  n
----------  ------------------------------  ---
[-8%, -6%)  ████████                        1
[-6%, -4%)                                  0
[-4%, -2%)                                  0
[-2%, +0%)  ██████████████████████████████  4

transform_yield_count:

• Average change: +0.2
• Average regression: +1.0
• Median change: +1
• Median regression: +1
• Standard deviation: 1.6
• Standard deviation of regressions: 0.0
• Range: -3 to +1
• Range of regressions: +1 to +1
• Queries changed: 5
• Queries regressed: 4

transform_yield_count % change distribution (5 queries, bin = 2%):

     Range                                  n
----------  ------------------------------  ---
[-6%, -4%)  ████████                        1
[-4%, -2%)                                  0
[-2%, +0%)                                  0
[+0%, +2%)                                  0
[+2%, +4%)  ██████████████████████████████  4

insert_new_count:

• Average change: -3.8
• Median change: -4
• Standard deviation: 0.4
• Range: -4 to -3
• Queries changed: 5
• No regressions! 🎉

insert_new_count % change distribution (5 queries, bin = 2%):

     Range                                  n
----------  ------------------------------  ---
[-8%, -6%)  ██████████████████████████████  4
[-6%, -4%)                                  0
[-4%, -2%)  ████████                        1

insert_reused_count:

• Average change: -1.0
• Median change: -1
• Standard deviation: 0.0
• Range: -1 to -1
• Queries changed: 1
• No regressions! 🎉

There were no queries with significant regressions detected.

Minor Changes (Plan and Metrics Changed)

In addition, there were 5 queries with minor changes.