fix: check cached file existence asynchronously before removing it

[yamashita-room-335]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix.

⤵️ What is the current behavior?

CacheStore._removeCachedFile calls the synchronous existsSync() before deleting a cached file. Cache cleanup, removeFile and emptyCache all go through it. On Android with Flutter 3.29 or later this blocks the platform thread and can cause ANRs.

🆕 What is the new behavior (if this is a feature change)?

The existence check uses await file.exists(). Because the check is now asynchronous, two concurrent removals of the same entry can both see the file and race on delete(). A failed delete is ignored when the file no longer exists and rethrown otherwise. Before, only PathNotFoundException was ignored, which MemoryFileSystem does not throw. The version is bumped to 3.4.6.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

dart format ., flutter analyze and flutter test pass locally on Flutter 3.47.7. The new test in cache_store_test.dart uses a File whose existsSync() throws and fails without the fix. The existing test "Store should return null if file is not cached" covers the concurrent removal.

📝 Links to relevant issues/docs

Fixes #539

🤔 Checklist before submitting

• All projects build (the example app was not built)
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current main
• Version bumped and dated CHANGELOG entry added