assistant: Add tool registry (#17331)

This PR adds a tool registry to hold tools that can be called by the
Assistant.

Currently we just have a `now` tool for retrieving the current datetime.

This is all behind the `assistant-tool-use` feature flag which currently
needs to be explicitly opted-in to in order for the LLM to see the
tools.

Release Notes:

- N/A

crates/assistant/src/context.rs

@@ -9,9 +9,11 @@ use anyhow::{anyhow, Context as _, Result};
 use assistant_slash_command::{
     SlashCommandOutput, SlashCommandOutputSection, SlashCommandRegistry,
 };
+use assistant_tool::ToolRegistry;
 use client::{self, proto, telemetry::Telemetry};
 use clock::ReplicaId;
 use collections::{HashMap, HashSet};
+use feature_flags::{FeatureFlag, FeatureFlagAppExt};
 use fs::{Fs, RemoveOptions};
 use futures::{
     future::{self, Shared},
@@ -27,7 +29,7 @@ use language::{AnchorRangeExt, Bias, Buffer, LanguageRegistry, OffsetRangeExt, P
 use language_model::{
     LanguageModel, LanguageModelCacheConfiguration, LanguageModelCompletionEvent,
     LanguageModelImage, LanguageModelRegistry, LanguageModelRequest, LanguageModelRequestMessage,
-    MessageContent, Role,
+    LanguageModelRequestTool, MessageContent, Role,
 };
 use open_ai::Model as OpenAiModel;
 use paths::{context_images_dir, contexts_dir};
@@ -1942,7 +1944,21 @@ impl Context {
         // Compute which messages to cache, including the last one.
         self.mark_cache_anchors(&model.cache_configuration(), false, cx);
 
-        let request = self.to_completion_request(cx);
+        let mut request = self.to_completion_request(cx);
+
+        if cx.has_flag::<ToolUseFeatureFlag>() {
+            let tool_registry = ToolRegistry::global(cx);
+            request.tools = tool_registry
+                .tools()
+                .into_iter()
+                .map(|tool| LanguageModelRequestTool {
+                    name: tool.name(),
+                    description: tool.description(),
+                    input_schema: tool.input_schema(),
+                })
+                .collect();
+        }
+
         let assistant_message = self
             .insert_message_after(last_message_id, Role::Assistant, MessageStatus::Pending, cx)
             .unwrap();
@@ -2788,6 +2804,16 @@ pub enum PendingSlashCommandStatus {
     Error(String),
 }
 
+pub(crate) struct ToolUseFeatureFlag;
+
+impl FeatureFlag for ToolUseFeatureFlag {
+    const NAME: &'static str = "assistant-tool-use";
+
+    fn enabled_for_staff() -> bool {
+        false
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct PendingToolUse {
     pub id: String,