fix: rely on parent flex gap for Field helper-message spacing (#5080)

* fix: align helper-message spacing token across all Field-based inputs

Switch .eds-field__helper-message from --eds-spacing-vertical-3xs to
--eds-selectable-space-horizontal so all Field-based inputs (TextField,
Select, Search, Autocomplete, Checkbox, Radio, Switch) use the same
density-aware token as TextArea's .helper-row.

Closes #5042

* fix: remove redundant helper-message margin overrides

The calc resolves to 0 (--eds-selectable-space-horizontal equals
--eds-generic-gap-vertical), so the overrides on .eds-field__helper-message
in field.css and on .helper-row + .eds-field__helper-message in textarea.css
are no-ops. The helper-message already gets the correct 8px gap from the
parent .eds-field flex gap.

Addresses PR review feedback on #5080.

Author: pomfrida

packages/eds-core-react/src/components/next/Field/field.css

@@ -41,8 +41,8 @@
     display: block;
 
     font-size: var(--eds-typography-ui-body-md-font-size);
-    line-height: var(--eds-typography-ui-body-md-line-height-default);
     font-weight: var(--eds-typography-ui-body-md-font-weight-normal);
+    line-height: var(--eds-typography-ui-body-md-line-height-default);
     color: var(--_eds-field-label-color);
   }
 
@@ -62,11 +62,6 @@
 
   .eds-field__helper-message {
     margin: 0;
-    /* Tighten spacing above helper-message to 3xs (overrides parent gap).
-       Longhand follows shorthand to win the cascade. */
-    margin-block-start: calc(
-      var(--eds-spacing-vertical-3xs) - var(--eds-generic-gap-vertical)
-    );
     color: var(--_eds-field-helper-color);
   }
 

packages/eds-core-react/src/components/next/TextArea/textarea.css

@@ -39,13 +39,6 @@
       gap: var(--eds-typography-gap-horizontal);
       align-items: baseline;
       inline-size: 100%;
-      margin-block-start: calc(
-        var(--eds-selectable-space-horizontal) - var(--eds-generic-gap-vertical)
-      );
-    }
-
-    & .eds-field__helper-message {
-      margin-block-start: 0;
     }
 
     & .char-count {