fix(DatePicker): make underline variant border override order-independent

The underline variant relied on datepicker.module.scss's
.picker-underline { border-color: transparent } outranking the base
.picker { border: var(--picker-border) } from ocpicker.module.scss.
Both selectors have equal specificity, so the winner was decided by
CSS emission order in the consuming app's webpack build — which is
derived from the app-wide import graph and can flip on unrelated
changes (mini-css-extract conflicting-order resolution). When it
flips, the box border reappears around underline-sized content and
clips the suffix calendar icon.

Declare border-color: transparent inside ocpicker.module.scss's own
.picker-underline block, where same-file source order guarantees it
always follows the base .picker rule regardless of bundler ordering.
Covers DatePicker, TimePicker and RangePicker (shared internal styles).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: agupta-eightfold

src/components/DateTimePicker/Internal/ocpicker.module.scss

@@ -9,7 +9,7 @@
   border-radius: $picker-border-radius;
   display: inline-flex;
   font-family: var(--font-stack-full);
-  
+
   .pickerLabel {
     position: relative;
   }
@@ -1705,6 +1705,12 @@
   }
 
   &-underline {
+    // Must live in this file: the box border it hides is set by `.picker`
+    // above, at equal specificity. Cross-file overrides (datepicker.module.scss)
+    // depend on the consuming app's webpack CSS emission order, which is not
+    // guaranteed — same-file source order is.
+    border-color: transparent;
+
     .picker-input {
       &:before {
         border-bottom: 1px solid var(--input-border-color);