geo: avoid false negatives for geography ST_DWithin

[Alignyx]

Summary

Prevent false negatives when a geography ST_DWithin query uses the geography inverted index. The fix keeps the inverted scan and exact residual predicate, but removes two lossy bounding-box assumptions that can discard a true match before exact evaluation.

Fixes #80374

What was wrong

Geography is represented on the sphere, and a polygon can describe the large side of its boundary. Its coordinate bounding box does not necessarily contain the represented spherical region. The DWithin covering path used a bounding-box fallback in this situation, so the generated S2 spans could omit cells that contain a qualifying indexed value.

The optional geography pre-filter made the same assumption: it derived a bounding rectangle and rejected candidates from that rectangle before the exact ST_DWithin residual predicate ran. For a large-side geography polygon, that rejection is not conservative and can turn a true result into a false negative.

Implementation

• Replace the bounding-box fallback in s2GeographyIndex.DWithin with the covering produced from the actual S2 region representation.
• Add a single relationship/type guard for pre-filter construction. The guard disables the optional pre-filter only for Geography plus DWithin.
• Apply the guard at both production construction sites: inverted filter extraction and the geography inverted-expression builder.

Other geometry relationships and other geography relationships retain their existing pre-filter behavior. The inverted index scan remains in the plan for geography DWithin; only the unsafe bounding-cap pruning is removed. Any extra candidates admitted by the conservative spans are still checked by the exact residual predicate.

Why this fixes the reported case

The query-side S2 covering now follows the represented spherical region rather than a coordinate rectangle that may describe only the short side of a polygon boundary. The optional pre-filter can no longer reject a geography DWithin candidate using that non-conservative rectangle. This removes the false-negative path while preserving exact result filtering and index-based execution.

Validation

• Built the affected geography index and query-observation targets.
• Ran the geography index package tests.
• Checked the reported intersection/DWithin cases, preserved relationships, and held-out large-side geography cases.
• Verified that the optimizer continues to produce inverted-index scans with the exact residual predicate where applicable.
• Ran the configured full regression and compared it with the baseline; all configured checks passed.

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