test: Add descriptive diag messages to all test_ffto_* tests

Add multi-line descriptive messages at startup for:
- test_ffto_bypass-t.cpp: Tests FFTO bypass for large queries
- test_ffto_mysql-t.cpp: Tests FFTO for MySQL connections
- test_ffto_pgsql-t.cpp: Tests FFTO for PostgreSQL connections

Explains FFTO (Fast Forward To Optimization) purpose and what
each test validates regarding query digest tracking.

test/tap/tests/test_ffto_bypass-t.cpp

@@ -17,6 +17,14 @@ int main(int argc, char** argv) {
 
     plan(2);
 
+    diag("=== FFTO Bypass Test ===");
+    diag("This test validates that queries larger than the FFTO max buffer size");
+    diag("bypass the FFTO (Fast Forward To Optimization) mechanism entirely.");
+    diag("When a query exceeds the threshold, it should NOT be recorded in");
+    diag("stats_mysql_query_digest since FFTO processing is skipped.");
+    diag("Test sets mysql-ffto_max_buffer_size=100 bytes and sends a 200+ byte query.");
+    diag("========================");
+
     MYSQL* admin = mysql_init(NULL);
     if (!mysql_real_connect(admin, cl.host, cl.admin_username, cl.admin_password, NULL, cl.admin_port, NULL, 0)) {
         diag("Admin connection failed");

test/tap/tests/test_ffto_mysql-t.cpp

@@ -80,6 +80,15 @@ int main(int argc, char** argv) {
         return -1;
     }
 
+    diag("=== FFTO MySQL Test ===");
+    diag("This test validates FFTO (Fast Forward To Optimization) for MySQL.");
+    diag("FFTO enables fast_forward mode where queries are passed directly to");
+    diag("the backend without full result set buffering in ProxySQL.");
+    diag("Tests verify query digests are recorded in stats_mysql_query_digest");
+    diag("with correct count_star, sum_rows_affected, and sum_rows_sent metrics");
+    diag("for CREATE, INSERT, SELECT, UPDATE, and DELETE operations.");
+    diag("=======================");
+
     plan(kPlannedTests); // 1 + 18 + 3 = 22
 
     MYSQL* admin = mysql_init(NULL);

test/tap/tests/test_ffto_pgsql-t.cpp

@@ -81,7 +81,17 @@ int main(int argc, char** argv) {
         return -1;
     }
 
-    plan(kPlannedTests); // 1 (connect) + 18 (simple) + 3 (extended) = 22
+    plan(kPlannedTests); // 1 (connect) + 18 (simple) + 3 (extended) = 19
+
+    diag("=== FFTO PostgreSQL Test ===");
+    diag("This test validates FFTO (Fast Forward To Optimization) for PostgreSQL.");
+    diag("FFTO enables fast_forward mode where queries are passed directly to");
+    diag("the backend without full result set buffering in ProxySQL.");
+    diag("Tests cover both Simple Query Protocol (using ? placeholders) and");
+    diag("Extended Query Protocol (using $1, $2 named placeholders via PQprepare).");
+    diag("Verifies query digests are recorded with correct count_star,");
+    diag("sum_rows_affected, and sum_rows_sent metrics in stats_pgsql_query_digest.");
+    diag("============================");
 
     MYSQL* admin = mysql_init(NULL);
     PGconn* conn = NULL;