fix(modal): fix enter animation when the initial breakpoint value is less than the backdrop breakpoint (#29955)

Issue number: internal

---------

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## What is the current behavior?
This PR aims to fix an issue introduced in
https://github.com/ionic-team/ionic-framework/pull/29932 where the enter
animation for ionic theme was in a broken state when the modal's initial
breakpoint value was lesser than the specified backdrop breakpoint.

## What is the new behavior?
- The erroneous behaviour happened because the shouldShowBackdrop
constant had a false value which was being taken into consideration when
computing the backdrop opacity for the ionic theme. This PR fixes this
by always transitioning the backdrop opacity from 0 to 0.7 once the
modal appears, independently of the initial, current or activation
breakpoint.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

core/src/components/modal/animations/sheet.ts

@@ -12,9 +12,12 @@ export const createSheetEnterAnimation = (opts: ModalAnimationOptions) => {
    * current breakpoint, then the backdrop should be fading in.
    */
   const shouldShowBackdrop = backdropBreakpoint === undefined || backdropBreakpoint < currentBreakpoint!;
-  const initialBackdrop = shouldShowBackdrop
-    ? `calc(var(--backdrop-opacity) * ${staticBackdropOpacity ? 1 : currentBreakpoint!})`
-    : '0';
+  let initialBackdrop = '0';
+  if (staticBackdropOpacity) {
+    initialBackdrop = 'calc(var(--backdrop-opacity)';
+  } else if (shouldShowBackdrop) {
+    initialBackdrop = `calc(var(--backdrop-opacity) * ${currentBreakpoint!})`;
+  }
 
   const backdropAnimation = createAnimation('backdropAnimation').fromTo('opacity', 0, initialBackdrop);
 

core/src/components/modal/modal.tsx

@@ -1049,6 +1049,8 @@ interface ModalOverlayOptions {
   /**
    * The point after which the backdrop will begin
    * to fade in when using a sheet modal.
+   * This option is ignored for the ionic theme
+   * due to the option below.
    */
   backdropBreakpoint: number;
   /**