fix(langgraph/python): import ag_ui_langgraph without FastAPI installed (#2067)

[dkedar7]

Fixes #2067.

Problem

FastAPI is declared an optional dependency of ag-ui-langgraph (only the HTTP endpoint helper in endpoint.py uses it), but ag_ui_langgraph/__init__.py eagerly imports .endpoint. So a purely in-process consumer — one that just iterates LangGraphAgent.run() to render AG-UI events locally (a terminal or Jupyter renderer, say) — can't import the package without the web stack:

pip install ag-ui-langgraph langgraph      # note: NOT the [fastapi] extra
python -c "from ag_ui_langgraph import LangGraphAgent"
# ModuleNotFoundError: No module named 'fastapi'
#   File ".../ag_ui_langgraph/__init__.py", line 20, in <module>
#     from .endpoint import add_langgraph_fastapi_endpoint

endpoint.py is the only module importing fastapi/starlette; the adapter itself (agent.py, types.py, utils.py, a2ui_tool.py, middlewares/) is already FastAPI-free.

Fix

Guard the optional endpoint import:

try:
    from .endpoint import add_langgraph_fastapi_endpoint
except ImportError:
    # FastAPI is an optional dependency — only the HTTP endpoint helper needs it.
    add_langgraph_fastapi_endpoint = None

add_langgraph_fastapi_endpoint stays in __all__ and is simply None until FastAPI is present, so the served path is unchanged.

Tests

Adds tests/test_import_without_fastapi.py — a regression test that blocks fastapi/starlette in a subprocess and asserts LangGraphAgent imports and add_langgraph_fastapi_endpoint is None. Verified the existing test_deprecation_warnings.py and test_endpoint_health_path.py still pass (the guard doesn't touch the served path), and the full Python suite passes (380 tests).

_{Context: I filed #2067 two weeks ago with this diagnosis and offered to PR; sending it now since the change is small and ready. Happy to adjust to whatever you prefer.}