langgraph-api doesn't support dict format for graph specification

[gfortaine]

Hi team,

PR #4164 updated the LangGraph CLI to allow specifying graphs in langgraph.json using a dictionary format:

{
  "dependencies": ["./my_agent"],
  "graphs": {
    "agent": {
      "path": "./my_agent/agent.py:graph",
      "description": "this is my agent description"
    }
  },
  "env": ".env"
}

However, the langgraph-api package, specifically the collect_graphs_from_env function in langgraph_api/graph.py, was not updated to handle this new format. It still assumes the value for each graph is a string (path:variable).

This leads to the following error when langgraph-api tries to parse a langgraph.json using the dictionary format:

Traceback (most recent call last):
  File ".../langgraph_api/lifespan.py", line 49, in lifespan
    await collect_graphs_from_env(True)
  File ".../langgraph_api/graph.py", line 198, in collect_graphs_from_env
    path_or_module, variable = value.rsplit(":", maxsplit=1)
                               ^^^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'rsplit'

The collect_graphs_from_env function needs to be updated to check the type of the value and extract the path string if it's a dictionary.

Here's a suggested implementation for the relevant part of the function:

async def collect_graphs_from_env(register: bool = False) -> None:
    # ... (previous code) ...

    if paths_str:
        specs = []
        for key, value in json.loads(paths_str).items():
            # Check if value is a dictionary (new format) or string (old format)
            if isinstance(value, dict):
                path_string = value.get("path")
                if not path_string or not isinstance(path_string, str):
                    raise ValueError(
                        f"Invalid dictionary format for graph '{key}'. Missing or invalid 'path' key."
                    )
            elif isinstance(value, str):
                path_string = value
            else:
                raise ValueError(
                    f"Invalid format for graph '{key}'. Expected string or dictionary, got {type(value)}."
                )

            try:
                path_or_module, variable = path_string.rsplit(":", maxsplit=1)
            except ValueError as e:
                raise ValueError(
                    f"Invalid path string '{path_string}' for graph '{key}'."
                    " Did you miss a variable name?\n"
                    " Expected format: 'path/to/file.py:variable_name' or 'my.module:variable_name'"
                ) from e

            # Determine if it's a module path or file path based on the presence of '/'
            is_module_path = "/" not in path_or_module
            specs.append(
                GraphSpec(
                    key,
                    module=path_or_module if is_module_path else None,
                    path=None if is_module_path else path_or_module,
                    variable=variable,
                    config=config_per_graph.get(key),
                )
            )
    # ... (rest of the function) ...

Could you please update langgraph-api to align with the CLI changes? Thanks!