improvement(ci): route maintenance tests in backend IT

[FANNG1]

What changes\n- trigger Backend Integration Test for changes\n- detect maintenance-only changes in backend IT workflow\n- run only maintenance integration tests for maintenance-only changes\n- run existing backend IT together with maintenance tests for non-maintenance-only changes\n\n## Why\n- verify backend IT routing behavior for maintenance changes\n\n## Test\n- workflow YAML parsed locally with Ruby Psych\n- prepared stacked validation branches for maintenance-only, mixed, and non-maintenance scenarios

[github-actions]

Code Coverage Report

Overall Project	64.87%	🟢
Files changed	No Java source files changed	-

Module	Coverage
aliyun	1.73%	🔴
api	47.14%	🟢
authorization-common	85.96%	🟢
aws	1.1%	🔴
azure	2.6%	🔴
catalog-common	10.0%	🔴
catalog-fileset	80.02%	🟢
catalog-hive	80.98%	🟢
catalog-jdbc-clickhouse	79.06%	🟢
catalog-jdbc-common	42.89%	🟢
catalog-jdbc-doris	80.28%	🟢
catalog-jdbc-hologres	54.03%	🟢
catalog-jdbc-mysql	79.23%	🟢
catalog-jdbc-oceanbase	78.38%	🟢
catalog-jdbc-postgresql	82.05%	🟢
catalog-jdbc-starrocks	78.27%	🟢
catalog-kafka	77.01%	🟢
catalog-lakehouse-generic	45.07%	🟢
catalog-lakehouse-hudi	79.1%	🟢
catalog-lakehouse-iceberg	87.15%	🟢
catalog-lakehouse-paimon	77.71%	🟢
catalog-model	77.72%	🟢
cli	44.51%	🟢
client-java	77.83%	🟢
common	49.42%	🟢
core	80.96%	🟢
filesystem-hadoop3	76.97%	🟢
flink	38.86%	🔴
flink-runtime	0.0%	🔴
gcp	14.2%	🔴
hadoop-common	10.39%	🔴
hive-metastore-common	45.82%	🟢
iceberg-common	50.21%	🟢
iceberg-rest-server	66.51%	🟢
integration-test-common	0.0%	🔴
jobs	66.17%	🟢
lance-common	23.88%	🔴
lance-rest-server	57.84%	🟢
lineage	53.02%	🟢
optimizer	82.95%	🟢
optimizer-api	21.95%	🔴
server	85.6%	🟢
server-common	69.43%	🟢
spark	32.79%	🔴
spark-common	39.09%	🔴
trino-connector	31.62%	🔴