fix: bound get_run_io() + implement gen-history null-wpdb test (#177, v0.27.1)

Closes the two M4 P2s: LIMIT 100 safety cap on the per-run stage-I/O query; implements the previously-stubbed get_page() null-wpdb test. QA APPROVE; CI green.

Agent-Session: pipeline-p2-cleanup-merge

includes/admin/class-gen-history-query.php

@@ -160,7 +160,8 @@ class PRAutoBlogger_Gen_History_Query {
 					request_json, response_status, error_message, created_at
 				 FROM {$log_table}
 				 WHERE run_id = %s
-				 ORDER BY id ASC",
+				 ORDER BY id ASC
+				 LIMIT 100", // Safety cap: a run has ~6-10 stage rows; 100 is generous.
 				$run_id
 			),
 			ARRAY_A