Fix share sync update loss on flush failure

[OnlyYu1996]

Summary

• requeue the failed sync update plus any not-yet-sent updates when ShareSync::flush() hits a send error
• preserve successful sends so already-flushed updates are not retried
• add a regression test that simulates a mid-flush connection failure and verifies pending updates remain queued

Fixes PlatformNetwork/bounty-challenge#53166

Verification

• cargo fmt --package cortex-share --check
• cargo test --package cortex-share sync::tests::test_flush_requeues_failed_and_unsent_updates -- --nocapture
• cargo test --package cortex-share -- --nocapture
• cargo check --package cortex-share --lib
• git diff --check

[greptile-apps]

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