feat(adapters/langgraph): make agent structured output not hang or silently vanish

[spichen]

Problem

create_agent(response_format=...) ties the agent's exit condition to getting a parseable structured response. The adapter set it for any agent with declared outputs, so a model that won't produce one broke two ways.

With no tools the agent loops. create_agent sets recursion_limit=9999 where LangGraph's default is 25, so it burns thousands of model calls and then raises a GraphRecursionError naming neither the agent nor the schema.

With tools it's quieter and worse. Routing treats a turn with no tool calls as done, so the agent exits, structured_response is never set, and a two-output agent returns zero outputs with no error (langchain#36349, open).

Nothing upstream bounds this. ToolStrategy.handle_errors only covers malformed tool calls; True, False and a custom message all still hang. See also langgraph#6731 (closed, not planned) and this forum thread. Upgrading doesn't help: the behaviour is identical at both ends of the range the pins allow, checked on langchain 1.3.1/1.3.14, langchain-core 1.4.0/1.4.9 and langgraph 1.2.0/1.2.9. Same loop, same silent exit, same recursion_limit=9999.

Fix

An agent whose only declared output is a string needs no structured generation, so response_format is skipped and the value read from the final message. LlmNodeExecutor already worked this way; AgentNode now matches it.

Everything else keeps response_format plus a StructuredOutputGuard middleware. after_model bounds the loop by counting consecutive turns that end in an AIMessage with no tool calls. after_agent catches the silent exit, which that budget never sees. Turns with tool calls don't count, since under ToolStrategy the structured response is itself a tool call.

is_single_string_output moved to adapters/_utils.py, replacing two copies of the predicate in LlmNodeExecutor, one of them dead. AgentMiddleware now comes from _types.py as a LazyType, so the guard is a plain module-level class and the module still imports without the langgraph extra.

Testing

Against a model that ignores structured output:

	before	after
single string output	GraphRecursionError (9999 calls)	{'answer': '42'}
multi-field, no tools	GraphRecursionError (9999 calls)	error after 3 calls
multi-field, with a tool	silently returned 0 of 2 outputs	error
multi-field, compliant model	works	works

That last row is the one that matters: neither hook may fire on a model that does produce the response.

454 passing across tests/adapters, tests/serialization and tests/validation under SKIP_LLM_TESTS=1, pinned to constraints/constraints.txt; failures and skips unchanged from main. black, isort, pyflakes, mypy clean. The 10 new tests build no LLM config, so they run offline instead of skipping.