fix(compose): validate indirect edges in Workflow compile

[JopenChen]

What type of PR is this?

fix

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• The description of this PR title is user-oriented and clear enough for others to understand.
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fix(compose): 在 Workflow 编译时校验间接边的合法性

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
Add validation for noDirectDependency (indirect) edges during Workflow compilation.

There was a TODO at compose/workflow.go:509: "check indirect edges are legal". Edges created via WithNoDirectDependency() only establish data mappings without direct execution dependencies. Without validation, an illegal graph where no execution path exists from source to target could compile successfully and cause undefined behavior at runtime.

Changes:

• Add validateIndirectEdges() called before graph compilation
• Build control adjacency from g.controlEdges + g.branches (not wf.dependencies, because branchDependency can be overwritten by noDirectDependency for the same node pair)
• BFS reachability check for each noDirectDependency edge
• Clear error message for illegal configurations
• 4 test cases: 2 illegal scenarios + 2 valid scenarios (including branch paths)

zh(optional):
在 Workflow 编译阶段增加对间接边（noDirectDependency）的合法性校验。

compose/workflow.go:509 处原有 TODO 标注 "check indirect edges are legal"。通过 WithNoDirectDependency() 创建的边仅建立数据映射而无直接执行依赖。若不校验，源节点到目标节点不存在执行路径的非法图也能编译通过，运行时产生不可预期行为。

主要改动：

• 新增 validateIndirectEdges()，在图编译前调用
• 从 g.controlEdges 和 g.branches 构建控制邻接表（不使用 wf.dependencies，因为同一节点对的 branchDependency 可能被 noDirectDependency 覆盖）
• 对每条间接边执行 BFS 可达性检查
• 非法配置返回清晰的错误信息
• 新增 4 个测试用例：2 个非法场景 + 2 个合法场景（含分支路径）

(Optional) Which issue(s) this PR fixes:

N/A (resolves internal TODO)

(optional) The PR that updates user documentation:

N/A