fix(acp): support agent-client-protocol v0.9.0 schema changes (langchain-ai#2700)

Fixes langchain-ai#2678

`agent-client-protocol` v0.9.0 upgraded the ACP schema from v0.10.8 to
v0.11.2
([agentclientprotocol/python-sdk#80](agentclientprotocol/python-sdk#80)),
which removed the `SessionConfigOption` wrapper type. Config options are
now bare `SessionConfigOptionSelect` / `SessionConfigOptionBoolean`
instances. Since `deepagents-acp` pins `agent-client-protocol>=0.8.0`
with no upper bound, new installs resolve to v0.9.0 and hit
`ImportError: cannot import name 'SessionConfigOption' from
'acp.schema'` at startup.

Uses a conditional import to support both v0.8.x (with
`SessionConfigOption` wrapper) and v0.9.0+ (without wrapper). When the
wrapper is available, config options are constructed as
`SessionConfigOption(root=SessionConfigOptionSelect(...))`; when absent,
they are bare `SessionConfigOptionSelect(...)` instances.

All 76 existing ACP tests pass.

```
$ pytest tests/ -v
======================== 76 passed, 1 warning in 3.31s =========================
```

> This contribution was developed with AI assistance (Cursor).

---------

Co-authored-by: shengyi <shengyi.tang@shopee.com>
Co-authored-by: Jacob Lee <jacoblee93@gmail.com>
Co-authored-by: Vivek Trivedy <vivek.trivedy@langchain.dev>

Author: 3 people

libs/acp/deepagents_acp/server.py

@@ -39,7 +39,6 @@
     PlanEntry,
     PromptCapabilities,
     ResourceContentBlock,
-    SessionConfigOption,
     SessionConfigOptionSelect,
     SessionConfigSelectOption,
     SessionModeState,
@@ -49,6 +48,13 @@
     ToolCallUpdate,
     ToolKind,
 )
+
+try:
+    from acp.schema import SessionConfigOption
+except ImportError:
+    # agent-client-protocol >=0.9.0 removed the SessionConfigOption wrapper;
+    # config options are now bare SessionConfigOptionSelect instances.
+    SessionConfigOption = None  # type: ignore[assignment,misc]
 from deepagents import create_deep_agent
 from deepagents.backends import CompositeBackend, FilesystemBackend, StateBackend
 from langgraph.checkpoint.memory import MemorySaver
@@ -136,14 +142,17 @@ def on_connect(self, conn: Client) -> None:
         """Store the client connection for sending session updates."""
         self._conn = conn
 
-    def _build_config_options(self, session_id: str) -> list[SessionConfigOption]:
+    def _build_config_options(
+        self,
+        session_id: str,
+    ) -> list[SessionConfigOptionSelect]:
         """Build the list of session configuration options.
 
         Returns a list combining mode and model selectors if available.
         Modes are mapped to config options with category='mode'.
         Models are exposed as config options with category='model'.
         """
-        config_options: list[SessionConfigOption] = []
+        config_options: list[SessionConfigOptionSelect] = []
 
         # Add mode selector if modes are configured
         if self._modes is not None:
@@ -157,18 +166,18 @@ def _build_config_options(self, session_id: str) -> list[SessionConfigOption]:
                 for mode in self._modes.available_modes
             ]
 
-            mode_config = SessionConfigOption(
-                root=SessionConfigOptionSelect(
-                    id="mode",
-                    name="Session Mode",
-                    description="Controls how the agent requests permission",
-                    category="mode",
-                    type="select",
-                    current_value=current_mode,
-                    options=mode_options,
-                )
+            mode_select = SessionConfigOptionSelect(
+                id="mode",
+                name="Session Mode",
+                description="Controls how the agent requests permission",
+                category="mode",
+                type="select",
+                current_value=current_mode,
+                options=mode_options,
+            )
+            config_options.append(
+                SessionConfigOption(root=mode_select) if SessionConfigOption else mode_select,
             )
-            config_options.append(mode_config)
 
         # Add model selector if models are configured
         if self._models is not None and len(self._models) > 0:
@@ -182,18 +191,18 @@ def _build_config_options(self, session_id: str) -> list[SessionConfigOption]:
                 for model in self._models
             ]
 
-            model_config = SessionConfigOption(
-                root=SessionConfigOptionSelect(
-                    id="model",
-                    name="Model",
-                    description="The LLM model to use for this session",
-                    category="model",
-                    type="select",
-                    current_value=current_model,
-                    options=model_options,
-                )
+            model_select = SessionConfigOptionSelect(
+                id="model",
+                name="Model",
+                description="The LLM model to use for this session",
+                category="model",
+                type="select",
+                current_value=current_model,
+                options=model_options,
+            )
+            config_options.append(
+                SessionConfigOption(root=model_select) if SessionConfigOption else model_select,
             )
-            config_options.append(model_config)
 
         return config_options