fix(plugin-postgresql): resolve effective schema when public does not exist

[datlechin]

Fixes #1662

A PostgreSQL database with no public schema showed no tables and no schema selector. The user had to create an empty public schema to get the picker to appear.

Two defects combined:

• LibPQDriverCore.currentSchema was hardcoded to public and only updated when SELECT current_schema() returned non-NULL. PostgreSQL returns NULL there when neither public nor a $user schema exists (default search_path is "$user", public), so every metadata query ran against a schema that does not exist.
• The schema picker only rendered with more than one schema, so a single-schema database left no way to switch.

Changes:

• connect() now falls back when current_schema() is NULL: first current_schemas(false)[1] (the first existing search_path entry, which honors ALTER ROLE/DATABASE SET search_path), then the first user schema alphabetically. On fallback it issues SET search_path so editor queries resolve to the same schema the sidebar shows. Runs before onPostConnect, so the metadata connection pool inherits the right schema.
• The schema picker shows whenever at least one schema is known, still gated to schema-switching drivers.
• setSearchPath no longer appends , public when switching schemas. Note: unqualified calls to extension functions installed in public (uuid-ossp, PostGIS) now require qualification after switching to another schema.
• The database list table count now counts all user schemas instead of only public.

CockroachDB and Redshift share LibPQDriverCore but always have a public schema, so the fallback never triggers for them. No PluginKit types touched, no ABI change.

Tests: PostgreSQLSearchPathTests (updated for the new SET SQL), new PostgreSQLDefaultSchemaFallbackTests and SchemaPickerVisibilityTests. All pass locally; swiftlint lint --strict clean.

DBeaver had this same bug and fixed it the same way in 7.1.4 (reading the effective search_path instead of assuming public).

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve PostGIS lookup for spatial rendering

When a user switches to a non-public schema on a database where PostGIS is installed in public, this removes public from the session search path. The driver's own spatial rendering path still runs unqualified conversion SQL (PostGISSpatialRewrite.geometryConversionQuery / geographyConversionQuery use ST_AsEWKT(t::geometry)), so those internal PQexecParams conversions fail and the app falls back to raw EWKB hex for geometry/geography columns. Please either keep the extension schema available for these internal conversions or qualify/probe the PostGIS functions/types before dropping public.

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[datlechin]

Follow-up from self-review (4f922c8):

• Schema probe now distinguishes a failed query from a NULL result. A transient error on SELECT current_schema() keeps the prior schema instead of triggering the fallback chain and overwriting the search path.
• Fallback queries are injected per driver: Redshift uses its USAGE-filtered pg_namespace list, so the chosen default is always readable by the connected role. PostgreSQL and CockroachDB keep the information_schema.schemata list.
• CHANGELOG: the search-path behavior change moved to its own Changed entry with the extension-function caveat spelled out, separate from the Fixed entry.
• New tests: PostgreSQLSchemaProbeTests (5 cases for the failure/NULL/text mapping) and Redshift fallback ordering. All targeted suites pass, swiftlint strict clean.