Migrate datasources panel to the catalog children model

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Summary

Adapts the extension to marimo-team/marimo#9874, which unifies catalog browsing around a recursive Database.children tree and one lazy-load endpoint. That PR:

• replaces the flat Database.schemas list with a tagged children tree (schema / namespace / data_table nodes), using null buckets for deferred discovery; and
• replaces the sql-table-list-preview + sql-schema-list-preview notifications with a single catalog-children-preview.

Since our notification/schema types are generated from @marimo-team/openapi, that is a compile break here once .marimo-version is bumped to the release containing #9874 (Database.schemas, SqlTableListPreviewNotification, and the old op tags all disappear). This migrates the consumers.

Changes

• types.ts — drop SqlTableListPreviewNotification; add CatalogChildrenPreviewNotification and CatalogNode / SchemaNode / NamespaceNode / DataTableNode.
• KernelManager.ts — route catalog-children-preview; the now-removed sql-schema-list-preview no longer needs to be ignored (the exhaustive unreachable(operation) switch enforces full coverage).
• DatasourcesService.ts — databases now hold a recursive CatalogTreeNode tree instead of a flat schema map. normalizeCatalogNode walks schema.tables / namespace.children and flattens deferred (null) buckets to [] (the panel renders what the kernel has discovered; it does not drive lazy fetches). updateCatalogChildrenPreview skips error responses so a transient failure can't erase cached children.
• DatasourcesView.ts — generalized the fixed Connection → Database → Schema → Table view into a recursive Connection → Database → (Schema | Namespace)* → Table tree using path-prefix matching. Adds a marimoNamespace contextValue (no menus key on it today). In-memory datasets render unchanged.
• Tests — DatasourcesService.test.ts (the panel's first): tree normalization (nested namespaces, root-level tables), deferred-null flattening, the error-skip guard, and clearNotebook.

Verification

Verified by checking out the #9874 commit in a sibling marimo checkout (so the linked @marimo-team/openapi + frontend are consistent):

• tsc — clean
• vp test — 5/5 pass
• vp check (lint + format) — clean

⚠️ Merge ordering

This must land together with the .marimo-version bump to the release containing #9874. Against the currently pinned openapi package the new wire types don't exist yet, so CI here will not go green until that bump is in this branch.

🤖 Generated with Claude Code

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