collab: Temporarily add back GET /user endpoint for local development (#35358)

This PR temporarily adds back the `GET /user` endpoint to Collab since
we're still using it for local development.

Will remove it again once we update the local development process to
leverage Cloud.

Release Notes:

- N/A

crates/collab/src/api.rs

@@ -100,6 +100,7 @@ impl std::fmt::Display for SystemIdHeader {
 
 pub fn routes(rpc_server: Arc<rpc::Server>) -> Router<(), Body> {
     Router::new()
+        .route("/user", get(legacy_update_or_create_authenticated_user))
         .route("/users/look_up", get(look_up_user))
         .route("/users/:id/access_tokens", post(create_access_token))
         .route("/users/:id/refresh_llm_tokens", post(refresh_llm_tokens))
@@ -144,6 +145,51 @@ pub async fn validate_api_token<B>(req: Request<B>, next: Next<B>) -> impl IntoR
     Ok::<_, Error>(next.run(req).await)
 }
 
+#[derive(Debug, Deserialize)]
+struct AuthenticatedUserParams {
+    github_user_id: i32,
+    github_login: String,
+    github_email: Option<String>,
+    github_name: Option<String>,
+    github_user_created_at: chrono::DateTime<chrono::Utc>,
+}
+
+#[derive(Debug, Serialize)]
+struct AuthenticatedUserResponse {
+    user: User,
+    metrics_id: String,
+    feature_flags: Vec<String>,
+}
+
+/// This is a legacy endpoint that is no longer used in production.
+///
+/// It currently only exists to be used when developing Collab locally.
+async fn legacy_update_or_create_authenticated_user(
+    Query(params): Query<AuthenticatedUserParams>,
+    Extension(app): Extension<Arc<AppState>>,
+) -> Result<Json<AuthenticatedUserResponse>> {
+    let initial_channel_id = app.config.auto_join_channel_id;
+
+    let user = app
+        .db
+        .update_or_create_user_by_github_account(
+            &params.github_login,
+            params.github_user_id,
+            params.github_email.as_deref(),
+            params.github_name.as_deref(),
+            params.github_user_created_at,
+            initial_channel_id,
+        )
+        .await?;
+    let metrics_id = app.db.get_user_metrics_id(user.id).await?;
+    let feature_flags = app.db.get_user_flags(user.id).await?;
+    Ok(Json(AuthenticatedUserResponse {
+        user,
+        metrics_id,
+        feature_flags,
+    }))
+}
+
 #[derive(Debug, Deserialize)]
 struct LookUpUserParams {
     identifier: String,