fix(protocol): guard connection state access

[buyua9]

Summary

• protect Messenger.connectionState with a small RW mutex and route mailserver sync gating through accessor helpers
• protect messaging.Core.connectionState while preserving the offline/online reliability transition behavior
• add a focused concurrent accessor regression for the messenger connection-state helpers

Follow-up to #7350, which restored the mobile ConnectionChange propagation chain.

Why

ConnectionChanged can run while mailserver sync checks read the current connection state to decide whether expensive networks should be allowed. The restored state cache was read and written directly, which leaves the new connection-state path race-prone under concurrent network changes and store-node sync checks.

Validation

• git diff --check origin/develop...HEAD
• changed-file conflict marker scan: no matches in this PR's changed files
• go test not run locally because this environment does not have the Go toolchain installed (go: command not found)