Fix RetryableMountingLayerException crash in SurfaceMountingManager.addViewAt

Summary:
## Problem

The `addViewAt` method in `SurfaceMountingManager` crashes with a `RetryableMountingLayerException` when trying to find a `ViewState` for a child tag that doesn't exist in the `tagToViewState` map.

**Error:** `Unable to find viewState for tag 290. Surface stopped: false`

**Logview:** [252c85116a7ab5c4ec93ef3c3373cf9d](https://www.internalfb.com/logview/system_vros_crashes/252c85116a7ab5c4ec93ef3c3373cf9d)

## Root Cause

The `addViewAt` method uses `getViewState()` which throws `RetryableMountingLayerException` when a tag is not found. This can happen due to race conditions where a view's state is removed (e.g., during surface cleanup or view deletion) before the mount item that references it is executed.

## Fix

Replaced `getViewState()` calls with `getNullableViewState()` + null checks + soft exception logging + early return for both the parent and child tag lookups in `addViewAt`. This matches the existing pattern already used by `removeViewAt`, which handles the same scenario gracefully.

The fix:
- Uses `getNullableViewState(parentTag)` instead of `getViewState(parentTag)` for the parent view state lookup
- Uses `getNullableViewState(tag)` instead of `getViewState(tag)` for the child view state lookup
- Adds null check for the child view itself
- Logs soft exceptions with the `SURFACE_MOUNTING_MANAGER_MISSING_VIEWSTATE` category for monitoring
- Returns early instead of crashing, allowing the surface to continue operating

Changelog: [Android][Fixed] - Fixed crash in SurfaceMountingManager.addViewAt when viewState is missing for a tag

Differential Revision: D99364621

Author: Improbability42

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/SurfaceMountingManager.kt

@@ -307,8 +307,7 @@ internal constructor(
       )
       return
     }
-    val view = viewState.view
-    checkNotNull(view) { "Unable to find view for viewState $viewState and tag $tag" }
+    val view = viewState.view ?: return
 
     // Display children before inserting
     if (SHOW_CHANGED_VIEW_HIERARCHIES) {