fix(tabs): add fallback to select tab if router integration fails (#30599)

Issue number: resolves #30552

---------

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issue. -->

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

Something caused a timing shift in v8.6.0 that messed up the timing
required for react router to set the active tab ID. Currently, when the
router goes to set the tab ID, it's possibly too early and the tab may
not exist yet, causing it to go unset.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

This PR is a workaround that allows tabs to check when they're rendered
if a tab should be selected as a fallback for the router not setting
them. I don't think the tabs, in the long run, should be responsible for
this, but I think this is a good intermediate step until the upcoming
react router upgrade, when we can look into a better solution for react
router that may require less timing precision.

This PR also adds regression tests for React to make sure this doesn't
happen again without getting noticed.

## 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. -->

**Current dev build:**
```
8.7.2-dev.11754338216.1a548096
```

core/src/components/tabs/tabs.tsx

@@ -68,7 +68,27 @@ export class Tabs implements NavOutlet {
   componentWillRender() {
     const tabBar = this.el.querySelector('ion-tab-bar');
     if (tabBar) {
-      const tab = this.selectedTab ? this.selectedTab.tab : undefined;
+      let tab = this.selectedTab ? this.selectedTab.tab : undefined;
+
+      // Fallback: if no selectedTab is set but we're using router mode,
+      // determine the active tab from the current URL. This works around
+      // timing issues in React Router integration where setRouteId may not
+      // be called in time for the initial render.
+      // TODO(FW-6724): Remove this with React Router upgrade
+      if (!tab && this.useRouter && typeof window !== 'undefined') {
+        const currentPath = window.location.pathname;
+        const tabButtons = this.el.querySelectorAll('ion-tab-button');
+
+        // Look for a tab button that matches the current path pattern
+        for (const tabButton of tabButtons) {
+          const tabId = tabButton.getAttribute('tab');
+          if (tabId && currentPath.includes(tabId)) {
+            tab = tabId;
+            break;
+          }
+        }
+      }
+
       tabBar.selectedTab = tab;
     }
   }