Read column descriptions from the schema, remove _columnDescriptions

Describe the cleanup

SDLB no longer exports _columnDescriptions in exportedConfig.json. Column descriptions written with
@column in the DataObject markdown files are now applied as column comments — to the data catalog and to
the exported schema JSON — by DataObjectSchemaExporter. The UI should read them from the schema like any
other comment, and drop the separate _columnDescriptions path.

This is the UI side of smart-data-lake/smart-data-lake#1121, where table and column metadata moved out of the
SDLB run into a deployment step.

What changed on the SDLB side

• ConfigJsonExporter.enrichColumnDescription was removed, so dataObjects.<id>._columnDescriptions is gone
  from the exported config. --descriptionPath / --uploadDescriptions still exist and still upload the
  markdown files themselves.
• The parsing moved to a shared ColumnDescriptionParser, used by DataObjectSchemaExporter.
• DataObjectSchemaExporter --mode apply merges the @column descriptions over the comments in the
  exported schema (markdown wins) and writes the result to the catalog. The schema JSON that the UI reads
  therefore already contains the final, consolidated comment in each field's comment attribute.

Places to change

• src/components/ConfigExplorer/ElementDetails.tsx:118 — stop passing
  columnDescriptions={configObj?._columnDescriptions} to SchemaTab.
• src/components/ConfigExplorer/SchemaTab.tsx — drop the columnDescriptions prop (declared at :54, read at
  :93, :104, :114, :130) and use column.comment alone.
  Note numberSchemaTree (:130-133) currently concatenates the markdown description and the schema comment
  when both exist. That merge now happens in SDLB, with the markdown description overriding the schema comment,
  so the concatenation should simply go away rather than be reimplemented.
• src/components/ConfigExplorer/ConfigurationAccordions.tsx:98 — _columnDescriptions can be removed from
  propsToIgnore, since the attribute no longer appears in the config.

Compatibility

_columnDescriptions was an optional attribute, so the UI just needs to stop relying on it. Configs exported by
an older SDLB will still contain it; ignoring it is fine.

One behavioural note

SDLB maps a markdown column path to the SQL column path, dropping array markers: b.[].b1 becomes b.b1, which
is the struct field inside the array — the common case, and it works. The UI additionally supports descriptions
attached to the array/map pseudo-rows themselves (-element-, -key-, -value-, addressed as a.[], a.key,
a.value). Those pseudo-nodes have no comment of their own in the schema JSON, so a description written
specifically for them is no longer displayed. Worth confirming whether anyone relies on that.