Factor tool definitions out of assistant (#21189)

This PR factors the tool definitions out of the `assistant` crate so
that they can be shared between `assistant` and `assistant2`.

`ToolWorkingSet` now lives in `assistant_tool`. The tool definitions
themselves live in `assistant_tools`, with the exception of the
`ContextServerTool`, which has been moved to the `context_server` crate.

As part of this refactoring I needed to extract the
`ContextServerSettings` to a separate `context_server_settings` crate so
that the `extension_host`—which is referenced by the `remote_server`—can
name the `ContextServerSettings` type without pulling in some undesired
dependencies.

Release Notes:

- N/A

crates/context_server/src/extension_context_server.rs

@@ -0,0 +1,77 @@
+use std::sync::Arc;
+
+use extension::{Extension, ExtensionContextServerProxy, ExtensionHostProxy, ProjectDelegate};
+use gpui::{AppContext, Model};
+
+use crate::{ContextServerFactoryRegistry, ServerCommand};
+
+struct ExtensionProject {
+    worktree_ids: Vec<u64>,
+}
+
+impl ProjectDelegate for ExtensionProject {
+    fn worktree_ids(&self) -> Vec<u64> {
+        self.worktree_ids.clone()
+    }
+}
+
+pub fn init(cx: &mut AppContext) {
+    let proxy = ExtensionHostProxy::default_global(cx);
+    proxy.register_context_server_proxy(ContextServerFactoryRegistryProxy {
+        context_server_factory_registry: ContextServerFactoryRegistry::global(cx),
+    });
+}
+
+struct ContextServerFactoryRegistryProxy {
+    context_server_factory_registry: Model<ContextServerFactoryRegistry>,
+}
+
+impl ExtensionContextServerProxy for ContextServerFactoryRegistryProxy {
+    fn register_context_server(
+        &self,
+        extension: Arc<dyn Extension>,
+        id: Arc<str>,
+        cx: &mut AppContext,
+    ) {
+        self.context_server_factory_registry
+            .update(cx, |registry, _| {
+                registry.register_server_factory(
+                    id.clone(),
+                    Arc::new({
+                        move |project, cx| {
+                            log::info!(
+                                "loading command for context server {id} from extension {}",
+                                extension.manifest().id
+                            );
+
+                            let id = id.clone();
+                            let extension = extension.clone();
+                            cx.spawn(|mut cx| async move {
+                                let extension_project =
+                                    project.update(&mut cx, |project, cx| {
+                                        Arc::new(ExtensionProject {
+                                            worktree_ids: project
+                                                .visible_worktrees(cx)
+                                                .map(|worktree| worktree.read(cx).id().to_proto())
+                                                .collect(),
+                                        })
+                                    })?;
+
+                                let command = extension
+                                    .context_server_command(id.clone(), extension_project)
+                                    .await?;
+
+                                log::info!("loaded command for context server {id}: {command:?}");
+
+                                Ok(ServerCommand {
+                                    path: command.command,
+                                    args: command.args,
+                                    env: Some(command.env.into_iter().collect()),
+                                })
+                            })
+                        }
+                    }),
+                )
+            });
+    }
+}