feat(search): geohash aggregation (bleve + opensearch)

[dschmidt]

Adds a geohash-grid aggregation to search, so a client can get map-cell density (bucket key = geohash cell, value = doc count) across all accessible spaces in one query.

New optional AggregationOption.geohash_precision (1-12). Clients aggregate on location; the server resolves it to the indexed geo field and rejects non-geo fields.

• OpenSearch: native geohash_grid on the geo-point sibling.
• bleve (no native geohash): index precomputes geohash-prefix keyword siblings (location_geohash_1..12) and runs a terms facet on the requested length. Same cells and counts as OpenSearch.

No schema-version bump: the mapping change is additive, so the existing reconcile extends the index in place and warns to reindex; no fresh index needed.

Stacked on #3211 (graph search query + aggregations). Independent of geo-KQL (#3212): the only shared helper (ResolveGeoField) is included here.

[codacy-production]

Not up to standards ⛔

🟢 Coverage 74.05% diff coverage

Metric	Results
Coverage variation	Report missing for bda62d71
Diff coverage	✅ 74.05% diff coverage

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Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (bda62d7)	Report Missing	Report Missing	Report Missing
Head commit (59aacc8)	84453	20148	23.86%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3272)	158	117	74.05%

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