Preserve declared StateGraph edge order in serialized graph output

[David Ahmann (davidahmann)]

Problem

StateGraph serialization can lose declaration order for equal-constraint edges, causing unstable output and ordering drift in downstream tooling.

Why now

Deterministic graph serialization is required for reproducible behavior and stable contract tests.

What changed

• Switched StateGraph edge storage to insertion-preserving lists.
• Added _edge_sequence tracking to preserve global edge declaration order.
• Ensured compile path applies edges in declared order.
• Updated CompiledStateGraph.get_graph() / aget_graph() to preserve declared non-conditional edge order while keeping conditional-edge positions stable.
• Added regression test test_state_graph_serialization_preserves_declared_edge_order.

Validation

• uv run pytest -q tests/test_pregel.py::test_state_graph_serialization_preserves_declared_edge_order
• uv run pytest -q tests/test_pregel.py::test_conditional_entrypoint_to_multiple_state_graph tests/test_pregel.py::test_conditional_state_graph_with_list_edge_inputs tests/test_pregel.py::test_state_graph_serialization_preserves_declared_edge_order

Refs #6979

[David Ahmann (davidahmann)]

Implemented the scoped fix for #6979 by preserving declared edge order in StateGraph serialization and adding a regression test that asserts deterministic declared-order output.

Validation run:

• (pass)

Current CI snapshot: reports no checks; is empty, so this is currently .

This contribution was informed by patterns from Wrkr. Wrkr scans your GitHub repo and evaluates every AI dev tool configuration against policy: https://github.com/Clyra-AI/wrkr