fix(test/tap): halve expected backend conn count in test_sqlite3_pass_exts-t on 9.x

Follow-up to cbbd10ba7 (skip native_password backend fixtures on MySQL 9.x).

The Phase-3 end-to-end loop runs RAND_USERS_GEN (100) iterations on 8.4
— 50 with mysql_native_password + 50 with caching_sha2_password — and
each successful login creates one backend connection. On 9.x the native
half is skipped, so only 50 actual connections are made.

The post-loop assertion "Number of backend conns created should match
conn attempts" was hardcoded to compare against RAND_USERS_GEN. When
native was skipped, actual=50 but expected=100, producing:

  not ok 2113 - Number of backend conns created should match conn
                attempts   exp:'100', act:'50'

Fix: compute expected_conns based on g_mysql_supports_native_password
(the same flag that gates the Phase-3 loop range) and use it both for
the wait_for_cond() condition string and the assertion.

Verified on a live mysql90 backend: test now reports
  ok 2113 - ... exp:'50', act:'50'
and exits 0 (2113/2113 assertions pass in 3.4s).

test/tap/tests/test_sqlite3_pass_exts-t.cpp

@@ -676,12 +676,19 @@ int main(int argc, char** argv) {
 			mysql_close(proxy);
 		}
 
+		// Only the iterations we actually ran produce backend connections.
+		// When the native_password half is skipped (MySQL 9.x), expected
+		// count is halved.
+		const uint32_t expected_conns =
+			g_mysql_supports_native_password ? RAND_USERS_GEN : (RAND_USERS_GEN / 2);
+		const string EXPECTED_CONNS_S { std::to_string(expected_conns) };
+
 		const string SEL_POOL_CONNS {
 			"SELECT SUM(ConnUsed + ConnFree) FROM stats.stats_mysql_connection_pool"
 				" WHERE hostgroup=" + TAP_MYSQL8_BACKEND_HG_S
 		};
 		const string COND_CONN_CREATION {
-			"SELECT IIF((" + SEL_POOL_CONNS + ")=" + RAND_USERS_GEN_S + ", 'TRUE', 'FALSE')"
+			"SELECT IIF((" + SEL_POOL_CONNS + ")=" + EXPECTED_CONNS_S + ", 'TRUE', 'FALSE')"
 		};
 		wait_for_cond(admin, COND_CONN_CREATION, 10);
 
@@ -693,9 +700,9 @@ int main(int argc, char** argv) {
 		}
 
 		ok(
-			RAND_USERS_GEN == cur_conns.val,
+			expected_conns == cur_conns.val,
 			"Number of backend conns created should match conn attempts   exp:'%u', act:'%lu'",
-			RAND_USERS_GEN, cur_conns.val
+			expected_conns, cur_conns.val
 		);
 	}