fix: only exclude nullable columns from DataTable PK when they have a default

Fixes a bug where non-nullable columns with DEFAULT constraints were
incorrectly removed from the DataTable PrimaryKey. This caused
ConstraintException when inserting rows that differ only in the column
that was wrongly excluded from the PK.

Root cause: the original condition checked only for the presence of a
DEFAULT constraint, but the comment described the intent as preventing
null PK values. For non-nullable columns, callers must always supply
the value, so including them in the PK is correct.

Fix: add the c.Nullable guard so that only nullable+default columns
are removed from the PK. Non-nullable columns with defaults retain
PartOfUniqueKey = true, so the generated DataTable constraint correctly
enforces uniqueness across all PK columns.

Closes #376

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/SqlClient.DesignTime/SqlClientProvider.fs

@@ -295,11 +295,13 @@ type SqlProgrammabilityProvider(config : TypeProviderConfig) as this =
                                 ?defaultValue = x.TryGetValue("default_constraint"), 
                                 ?description = x.TryGetValue("description")
                             ) 
-                        if c.DefaultConstraint <> "" && c.PartOfUniqueKey 
+                        if c.DefaultConstraint <> "" && c.Nullable && c.PartOfUniqueKey 
                         then 
                             { c with PartOfUniqueKey = false } 
-                            //ADO.NET doesn't allow nullable columns as part of primary key
-                            //remove from PK if default value provided by DB on insert.
+                            //ADO.NET doesn't allow null values in primary key columns.
+                            //Only remove nullable columns from PK when they have a default value,
+                            //since nullable+default columns may be omitted in NewRow() and remain null.
+                            //Non-nullable columns with defaults should stay in PK - callers must supply them.
                         else c
                     )
                     |> Seq.toList