fix(segment-button): update checked state on render (#26970)

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

In v7, asynchronously assigning a value to `ion-segment` and
`ion-segment-button` can cause the `ion-segment` to render without an
active value (segment button does not show as checked).

<!-- Issues are required for both bug fixes and features. -->
Issue URL: resolves #26830


## What is the new behavior?
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this PR. -->

- The `ion-segment-button` checked state is synced with the parent
`ion-segment`, when the component finishes the initial render.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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

Dev-build: `7.0.0-dev.11678736268.18a0beb1` ✅

---------

Co-authored-by: Liam DeBeasi <liamdebeasi@users.noreply.github.com>

core/src/components/segment-button/segment-button.tsx

@@ -1,5 +1,5 @@
 import type { ComponentInterface } from '@stencil/core';
-import { Component, Element, Host, Prop, Method, State, forceUpdate, h } from '@stencil/core';
+import { Component, Element, Host, Prop, Method, State, Watch, forceUpdate, h } from '@stencil/core';
 
 import { getIonMode } from '../../global/ionic-global';
 import type { ButtonInterface } from '../../utils/element-interface';
@@ -54,6 +54,10 @@ export class SegmentButton implements ComponentInterface, ButtonInterface {
    * The value of the segment button.
    */
   @Prop() value: string = 'ion-sb-' + ids++;
+  @Watch('value')
+  valueChanged() {
+    this.updateState();
+  }
 
   connectedCallback() {
     const segmentEl = (this.segmentEl = this.el.closest('ion-segment'));

core/src/components/segment/test/segment.spec.ts

@@ -16,3 +16,23 @@ it('should disable segment buttons added to disabled segment async', async () =>
   const segmentButton = page.body.querySelector('ion-segment-button');
   expect(segmentButton.disabled).toBe(true);
 });
+
+it('should set checked state when value is set asynchronously', async () => {
+  const page = await newSpecPage({
+    components: [Segment, SegmentButton],
+    html: `
+      <ion-segment value="first">
+        <ion-segment-button>First</ion-segment-button>
+      </ion-segment>
+    `,
+  });
+
+  const segmentButton = page.root.querySelector('ion-segment-button');
+
+  expect(segmentButton.classList.contains('segment-button-checked')).toBe(false);
+
+  segmentButton.value = 'first';
+  await page.waitForChanges();
+
+  expect(segmentButton.classList.contains('segment-button-checked')).toBe(true);
+});