fix(graphile-postgis): detect PostGIS when only geography codecs are present

[pyramation]

Summary

PostgisExtensionDetectionPlugin previously required a geometry codec to detect PostGIS. Databases that exclusively use geography columns (e.g. all DataPostGIS nodes with use_geography: true, as in agentic-db) never have a geometry codec introspected, so PostGIS detection silently failed. This produced warnings like:

Do not know how to convert PgCodec 'geography' into a GraphQL type

Root cause: The detection guard was if (!geometryCodec) return build — now changed to if (!geometryCodec && !geographyCodec) return build.

Changes across 4 files:

• types.ts: geometryCodec is now PgCodec | null (was PgCodec), symmetric with geographyCodec
• detect-extension.ts: Accept either codec for detection; extract schemaName from geography codec when geometry is absent
• connection-filter-operators.ts / within-distance-operator.ts: Conditionally push geometryCodec into codec pairs only when non-null (matches existing geographyCodec pattern)

Review & Testing Checklist for Human

• Verify no other consumers of PostgisExtensionInfo.geometryCodec outside this package assume it is non-null. The type changed from PgCodec to PgCodec | null — the build passes across the monorepo, but confirm no external packages depend on it being non-null at runtime.
• Verify register-types.ts line 97 — it already filters nulls via .filter((c): c is PgCodec => c !== null), but confirm this is the only place that iterates over both codecs.
• Test with agentic-db integration tests after this is published — run the integration test suite and confirm the geography codec warnings are gone and location_geo fields appear in the GraphQL schema.

Notes

• All 208 graphile-postgis unit tests and 35 preset-integration tests pass locally.
• Full monorepo build succeeds with no type errors.
• No new tests were added for the geography-only detection path specifically — existing tests cover the geometry+geography case and the no-PostGIS case. Consider adding a geography-only unit test to detect-extension.test.ts if desired.

Link to Devin session: https://app.devin.ai/sessions/6dd1a19fd02948fda1fee04c955f4bf2
Requested by: @pyramation

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