fix(tests): resolve flaky failures in pgsql-copy_from_stdin_session_parameter-t and test_dns_cache-t

1. pgsql-copy_from_stdin_session_parameter-t: race condition in log file reading

The test's check_logs_for_command() performed a single stream-based read
via get_matching_lines() to find the "Switching back to Normal mode" log
line. When ProxySQL hadn't flushed the log entry yet (observed window as
tight as ~23 microseconds between PQgetResult returning and the check),
the read would miss the line, causing a false test failure. The log line
was always present in the final file, confirming this is a timing issue.

Fix: Replace the single-shot read with a retry loop (up to 10 attempts,
10ms each = 100ms total). If all stream-based retries fail, fall back to
get_matching_lines_from_filename() which opens a fresh file handle,
avoiding any stream position issues entirely. Add a global
g_proxysql_log_path to support the filename-based fallback.

2. test_dns_cache-t: spurious dns_cache_queried counter increment in Step 9

Step 9 disables the DNS cache refresh interval (set to 0) and expects
dns_cache_queried to remain unchanged (equal_to). However, INVALID_DOMAIN
was still in mysql_servers from Step 8, so when "DO 1" queries were routed
through hostgroup 999, the connection routing layer called
MySQL_Monitor::dns_lookup() -> DNS_Cache::lookup(), incrementing the
counter by exactly 1 regardless of the refresh interval setting.

Fix: Before the measurement window in Step 9, replace INVALID_DOMAIN with
0.0.0.0 in mysql_servers. Since 0.0.0.0 is a valid IP address,
dns_lookup() returns immediately without consulting the DNS cache (early
return at MySQL_Monitor.cpp:6675), so the counter stays unchanged as
expected.

Both fixes verified: each test passes in isolation against legacy infra.

test/tap/tests/pgsql-copy_from_stdin_session_parameter-t.cpp

@@ -16,6 +16,7 @@
 CommandLine cl;
 
 constexpr unsigned int MAX_ITERATION = 10;
+std::string g_proxysql_log_path {};
 
 using PGConnPtr = std::unique_ptr<PGconn, decltype(&PQfinish)>;
 
@@ -103,8 +104,23 @@ bool sendCopyData(PGconn* conn, const char* data, int size, bool last) {
 }
 
 bool check_logs_for_command(std::fstream& f_proxysql_log, const std::string& command_regex) {
-    const auto& [_, cmd_lines] { get_matching_lines(f_proxysql_log, command_regex) };
-	return cmd_lines.empty() ? false : true;
+    // Retry with small delays to handle the race between ProxySQL writing
+    // the log line and the test reading it. The stream-based approach
+    // (get_matching_lines) can miss a line that hasn't been flushed yet.
+    constexpr int max_retries = 10;
+    constexpr useconds_t retry_delay_us = 10000; // 10ms
+    for (int i = 0; i < max_retries; i++) {
+        // First try the stream-based check (fast path, no filesystem hit)
+        const auto& [_, cmd_lines] { get_matching_lines(f_proxysql_log, command_regex) };
+        if (!cmd_lines.empty()) {
+            return true;
+        }
+        // Stream didn't find it yet — wait and retry from the file
+        usleep(retry_delay_us);
+    }
+    // Final attempt: read directly from file to avoid any stream position issues
+    const auto& [_, cmd_lines] { get_matching_lines_from_filename(g_proxysql_log_path, command_regex, false, 0) };
+    return !cmd_lines.empty();
 }
 
 bool setupTestTable(PGconn* conn) {
@@ -262,6 +278,7 @@ int main(int argc, char** argv) {
         return exit_status();
 
     std::string f_path{ get_env("REGULAR_INFRA_DATADIR") + "/proxysql.log" };
+    g_proxysql_log_path = f_path;
     std::fstream f_proxysql_log{};
 
     int of_err = open_file_and_seek_end(f_path, f_proxysql_log);

test/tap/tests/test_dns_cache-t.cpp

@@ -260,6 +260,11 @@ int main(int argc, char** argv) {
 			//disable dns cache
 			EXECUTE_QUERY("SET mysql-monitor_local_dns_cache_refresh_interval=0", proxysql_admin, false),
 			EXECUTE_QUERY("LOAD MYSQL VARIABLES TO RUNTIME", proxysql_admin, false),
+			// Replace INVALID_DOMAIN with an IP address so that routing
+			// lookups don't trigger DNS queries during the measurement window
+			EXECUTE_QUERY("DELETE FROM mysql_servers WHERE hostname='INVALID_DOMAIN'", proxysql_admin, false),
+			EXECUTE_QUERY("INSERT INTO mysql_servers (hostgroup_id,hostname,port,max_replication_lag,max_connections,comment) VALUES (999,'0.0.0.0',7861,0,1000,'dummy mysql server')", proxysql_admin, false),
+			EXECUTE_QUERY("LOAD MYSQL SERVERS TO RUNTIME", proxysql_admin, false),
 			DELAY_SEC(2),
 			UPDATE_PREV_METRICS(proxysql_admin),
 			LOOP_FUNC(EXECUTE_QUERY("DO 1", proxysql, true), 2),