fix(json-columns): skip JSON select columns with no model scan target

Requesting a JSON sub-field (e.g. jsonvalue->'product'->>'cost') that has
no matching bun scanonly field on the model made the whole read fail with
"bun: ModelX does not have column Y", since bun scans SELECT results
straight into the typed model struct.

Add reflection.HasColumn to check whether the model can actually receive
a given column (including scanonly fields, walking embedded structs), and
gate the JSON select-column expression on it in ApplySelectColumns
(shared by websocketspec/mqttspec) and the resolvespec/restheadspec
handlers. When there's no scan target, drop just that column with a
warning instead of erroring the whole request.

pkg/resolvespec/handler.go

@@ -349,6 +349,10 @@ func (h *Handler) handleRead(ctx context.Context, w common.ResponseWriter, id st
 			logger.Debug("Selecting columns: %v", options.Columns)
 			for _, col := range options.Columns {
 				if expr, jargs, alias, ok := common.ResolveJSONColumnExpr(model, "", col); ok {
+					if !reflection.HasColumn(model, alias) {
+						logger.Warn("Skipping JSON select column %q: model has no scan target for alias %q", col, alias)
+						continue
+					}
 					query = query.ColumnExpr(expr+" AS "+common.QuoteIdent(alias), jargs...)
 					continue
 				}