fix: add null check for match in async loader finally block

Fixes race condition where async loader's finally block executes after
navigation has changed, causing match to be undefined when accessing
_nonReactive properties.

This resolves 11 unhandled rejection errors in unit tests:
- TypeError: Cannot read properties of undefined (reading '_nonReactive')
- All errors pointed to load-matches.ts:851 in async callback's finally block

The fix adds a null check before accessing match._nonReactive to handle
cases where the match has been removed from the router during async execution.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: yanghuidong

packages/router-core/src/load-matches.ts

@@ -847,10 +847,13 @@ const loadRouteMatch = async (
             // Continue to resolve promises so head() can execute
           } finally {
             // Always resolve promises (success or error) to allow head() execution
-            const match = inner.router.getMatch(matchId)!
-            match._nonReactive.loaderPromise?.resolve()
-            match._nonReactive.loadPromise?.resolve()
-            match._nonReactive.loaderPromise = undefined
+            // Match might be undefined if navigation changed while async loader was running
+            const match = inner.router.getMatch(matchId)
+            if (match) {
+              match._nonReactive.loaderPromise?.resolve()
+              match._nonReactive.loadPromise?.resolve()
+              match._nonReactive.loaderPromise = undefined
+            }
           }
         })()
       } else if (status !== 'success' || (loaderShouldRunAsync && inner.sync)) {