Add Rust low-level tool-definition E2E test [5/6] (#1727)

* Add Rust low-level tool-definition E2E test

Related to issue #1682 but does not fix #1682.

Align low_level_tool_definition coverage with PR #1721 snapshot behavior by only defining tools exercised by the shared snapshot.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: revert session_lifecycle.rs Say hi -> Say world to match snapshot

The snapshot expects 'Say world' but the branch had changed it to 'Say hi',
causing 'No cached response found' failures across all three OS variants.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: edburns

rust/tests/e2e/tools.rs

@@ -4,10 +4,10 @@ use github_copilot_sdk::handler::{ApproveAllHandler, PermissionHandler, Permissi
 use github_copilot_sdk::tool::ToolHandler;
 use github_copilot_sdk::{
     Error, PermissionRequestData, RequestId, SessionConfig, SessionId, Tool, ToolInvocation,
-    ToolResult,
+    ToolResult, ToolSet,
 };
 use serde_json::json;
-use tokio::sync::mpsc;
+use tokio::sync::{Mutex, mpsc};
 
 use super::support::{assistant_message_content, recv_with_timeout, with_e2e_context};
 
@@ -73,6 +73,55 @@ async fn invokes_custom_tool() {
     .await;
 }
 
+#[tokio::test]
+async fn low_level_tool_definition() {
+    with_e2e_context("tools", "low_level_tool_definition", |ctx| {
+        Box::pin(async move {
+            ctx.set_default_copilot_user();
+            let client = ctx.start_client().await;
+            let __perm = Arc::new(ApproveAllHandler);
+            let current_phase = Arc::new(Mutex::new(String::new()));
+            let tools = vec![
+                set_current_phase_tool(current_phase.clone()),
+                search_items_tool(),
+            ];
+            let available_tools = ToolSet::new()
+                .add_custom("*")
+                .expect("add custom wildcard")
+                .add_builtin("web_fetch")
+                .expect("add web_fetch")
+                .into_vec();
+            let session = client
+                .create_session(
+                    SessionConfig::default()
+                        .with_github_token(super::support::DEFAULT_TEST_TOKEN)
+                        .with_permission_handler(__perm)
+                        .with_tools(tools)
+                        .with_available_tools(available_tools),
+                )
+                .await
+                .expect("create session");
+
+            let answer = session
+                .send_and_wait(
+                    "First, set the current phase to 'analyzing'. Then search for items with keyword 'copilot'. Report the phase and search results.",
+                )
+                .await
+                .expect("send")
+                .expect("assistant message");
+            let content = assistant_message_content(&answer);
+            assert!(!content.is_empty());
+            assert!(content.to_lowercase().contains("analyzing"));
+            assert!(content.contains("item_alpha") || content.contains("item_beta"));
+            assert_eq!(current_phase.lock().await.clone(), "analyzing");
+
+            session.disconnect().await.expect("disconnect session");
+            client.stop().await.expect("stop client");
+        })
+    })
+    .await;
+}
+
 #[tokio::test]
 async fn handles_tool_calling_errors() {
     with_e2e_context("tools", "handles_tool_calling_errors", |ctx| {
@@ -502,6 +551,69 @@ impl ToolHandler for ErrorTool {
 
 struct CustomGrepTool;
 
+struct SetCurrentPhaseTool {
+    current_phase: Arc<Mutex<String>>,
+}
+
+fn set_current_phase_tool(current_phase: Arc<Mutex<String>>) -> Tool {
+    Tool::new("set_current_phase")
+        .with_description("Sets the current phase of the agent")
+        .with_parameters(json!({
+            "type": "object",
+            "properties": {
+                "phase": {
+                    "type": "string",
+                    "description": "Current phase",
+                    "pattern": "^(searching|analyzing|done)$"
+                }
+            },
+            "required": ["phase"]
+        }))
+        .with_handler(Arc::new(SetCurrentPhaseTool { current_phase }))
+}
+
+#[async_trait::async_trait]
+impl ToolHandler for SetCurrentPhaseTool {
+    async fn call(&self, invocation: ToolInvocation) -> Result<ToolResult, Error> {
+        let phase = invocation
+            .arguments
+            .get("phase")
+            .and_then(serde_json::Value::as_str)
+            .unwrap_or_default()
+            .to_string();
+        *self.current_phase.lock().await = phase.clone();
+        Ok(ToolResult::Text(format!("Phase set to {phase}")))
+    }
+}
+
+struct SearchItemsTool;
+
+fn search_items_tool() -> Tool {
+    Tool::new("search_items")
+        .with_description("Search for items by keyword")
+        .with_parameters(json!({
+            "type": "object",
+            "properties": {
+                "keyword": { "type": "string" }
+            },
+            "required": ["keyword"]
+        }))
+        .with_handler(Arc::new(SearchItemsTool))
+}
+
+#[async_trait::async_trait]
+impl ToolHandler for SearchItemsTool {
+    async fn call(&self, invocation: ToolInvocation) -> Result<ToolResult, Error> {
+        let keyword = invocation
+            .arguments
+            .get("keyword")
+            .and_then(serde_json::Value::as_str)
+            .unwrap_or_default();
+        assert_eq!(keyword, "copilot");
+        Ok(ToolResult::Text("Found: item_alpha, item_beta".to_string()))
+    }
+}
+
 fn custom_grep_tool() -> Tool {
     Tool::new("grep")
         .with_description("A custom grep implementation that overrides the built-in")