MDEV-25817 proxy protocol: successful login does not reset connect errors

[vaintroub]

With proxy protocol thd_set_peer_addr() runs twice (proxy host, then the real client from the proxy header). Connect errors are accounted against the real client, but check_connection() incorrectly uses condition on the proxy host's count, rather than real client's address.

Fix: reset both the proxy host, and real client's connect errors on successful connection.

Added tests for incomplete handshake, and reset behavior, under proxy protocol, for both real client errors, and proxy host errors.

[gemini-code-assist]

Code Review

This pull request fixes an issue where successful logins under the PROXY protocol did not correctly reset the connect-error counters for the proxied client and the proxy host. It introduces a new flag have_proxy_protocol_connect_errors to track connection errors and ensures both the socket peer and the real client's error counters are reset upon a successful login. Additionally, test cases are added to verify this behavior. One review comment suggests a safer way to handle snprintf output length in the test code to prevent potential buffer over-reads.

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