fix(langgraph): Compile-time Detection of Detached Nodes

[f4roukb]

• Description: Introduce optional validation to warn or raise an exception on detached nodes. These are either unreachable or dead-ends not explicitly mapping to END. When developers specify the routing of their nodes, or that routing can be inferred from the node spec, it's used to detect detached nodes and handle it so that these don't go unnoticed. For developers that don't wish to specify the routing of the nodes (e.g., when routing by Command or conditional edges), they could set on_detached_nodes="ignore" to disable the node validation. Default would be "warn". Changes are backwards compatible.

• Issue: request: Compile-time validation of nodes that aren't statically linked #6735

• Dependencies: N/A.

• LinkedIn handle: linkedin.com/in/faroukb