fix(graph): prefer node tools before synthetic handoff routing

[andrewklatzke]

Summary

When a graph node has multiple outgoing edges, route() augments the node's config with synthetic __handoff_* tools. In practice this crowded out the node's own tools — a node that reliably called its real tool standalone would call only a handoff tool inside a graph, so the response carried routing information instead of the tool's data.

Three things in the routing augmentation pushed the model that way, all fixed here:

• The appended routing directive led with "Select exactly one transfer tool to route to the next agent", making the transfer read as the task and the node's real work optional. It now asks the model to complete its task with its available tools first, and only then call exactly one transfer tool.
• Handoff tool descriptions fell back to the target node's instructions verbatim, so __handoff_<target> advertised itself as the tool that does the target's work. Descriptions now always lead with Transfer control to <key>., with any handoff.description or target-instructions text appended as trailing detail.
• The handoff handler returned Transferring to <key>, which reads as though control had already left, so the model wrapped up instead of continuing. Selecting an edge only records the choice — execution continues until the model produces its final text — so the result now says the handoff is recorded and asks the model to finish its own work.

Behavior is unchanged for nodes with zero or one outgoing edge, which never enter this path. Framework-native runners build their own transfer_to_* tools and are untouched.

Test plan

• Existing graph suite passes (27 tests, packages/client/tests/test_graph.py)
• Verified against a real multi-node graph whose root node was skipping its tool: the node's own tool now fires and the handoff still routes correctly
• Reviewer sanity check on the routing copy — this is model-steering, so wording matters

Note: the matching change for the JS SDK is launchdarkly/js-ai-sdk#59