Fix for Issue #186: Prevent JSON queue file growth over 512 KB

[neelkanth-kaushik]

This pull request introduces proactive size checks on analytics-flutter-queue_flushing_plugin.json file so the SDK avoids queuing events that would push the on-disk queue file above 512 KB. When a new event would exceed the limit, the plugin attempts an immediate flush instead of silently trimming. Keep a safe trimming/fallback path on write failures or unexpected edge cases.

Fixed Github Issue: #186