[DSIP-64] Add more workflow graph integration test cases

[KaiSong-UK]

Purpose

Part of #16479 [DSIP-64] - Add more workflow Integration test cases in master.

Changes

Added 3 new workflow graph integration test cases in WorkflowStartMoreGraphTestCase.java:

1. Diamond fork-join pattern (A -> B, A -> C, B -> D, C -> D) - Classic fork-join where two parallel branches merge at a single downstream task.
2. Multi-level diamond pattern (A -> B -> C, A -> D -> E, C -> F, E -> F) - Each fork branch contains two serial tasks before merging.
3. Four serial tasks chain (A -> B -> C -> D) - Long serial pipeline with 4 linear tasks.

Each test case includes corresponding YAML workflow definitions under dolphinscheduler-master/src/test/resources/it/start/.

Verification

• All test cases verify the workflow instance reaches SUCCESS state
• All test cases assert each task instance reaches SUCCESS state
• Resource cleanup is verified via masterContainer.assertAllResourceReleased()

[SbloodyS]

Same as #18289 (comment)