fix(deepagents): exclude __pregel_ internal keys from subagent state to prevent Send serialization error

[Bob (zzzrbx)]

Summary

• LangGraph injects internal runtime objects (e.g. __pregel_send, a Send instance) into the graph state during execution
• These keys were being passed through to subagent state in _validate_and_prepare_state
• When LangGraph tried to checkpoint the subagent state, msgpack failed to serialize the Send object, crashing with: TypeError: Type is not msgpack serializable: Send
• Fix: filter out any key starting with __pregel_ alongside the existing _EXCLUDED_STATE_KEYS check

Changes

• deepagents/middleware/subagents.py: added _PREGEL_PREFIX = "__pregel_" constant and updated _validate_and_prepare_state to exclude matching keys
• tests/unit_tests/test_subagents.py: added TestPregelStateFiltering with a test that verifies __pregel_* keys do not appear in subagent state

Test plan

• All existing unit tests pass (make test)
• New test TestPregelStateFiltering::test_pregel_keys_excluded_from_subagent_state passes
• Reproduce: invoke a subagent that runs a tool call with a MemorySaver checkpointer — crash no longer occurs