fix(sidebar): list only the object sections that hold something

[datlechin]

Why

The sidebar listed a section for every object kind the engine supports, even when the schema held none. A PostgreSQL schema with no procedures, triggers or types showed a Procedures and a Triggers section reading "No items" and a Types section with nothing under it. The flat sidebar also always kept an empty Tables section.

Change

• A kind gets a section only when the schema holds objects of that kind, in both the flat and tree layouts. A search that matches nothing in a kind no longer leaves a bare header.
• A kind whose list the server refused keeps its section and shows the error there.
• A Tables section whose tables are all in folders stays, since dragging a table onto it takes it out of its folder.
• A schema that holds nothing shows one "No items" row, or "Loading…" while its lists arrive.

Verified

DatabaseTreeFlatRootTests builds the flat sidebar over a mocked PostgreSQL schema. It fails on main, where Procedures, Triggers and Types are listed. These tests pinned the old rule and are updated here: SidebarObjectKindTests (emptyTablesSectionIsFlatOnly, noContents, declaredKindShowsEmptySection), DatabaseTreeNodeTests.proceduresOnlySchemaGetsAGroup and DatabaseTreeFilterTests.declaredTypesKindIsVisible. Checked in a Debug build against PostgreSQL.

Screenshots

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Before
After

Not in this PR

• Redis: a sidebar search that matches only keys shows "No Results" instead of the Keys section.