fix: scope memory tools per-user in multi-tenant mode

Memory tools (search, write, read, tree) held a single workspace
created at startup with GATEWAY_USER_ID. In multi-tenant mode, all
users' tool calls searched the default user's scope.

Add WorkspaceResolver trait that resolves workspaces per-request using
JobContext.user_id. In single-user mode, returns the startup workspace.
In multi-tenant mode (GATEWAY_USER_TOKENS configured), creates and
caches per-user workspaces on demand.

Includes regression tests for workspace resolution and user isolation.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/extensions/manager.rs

@@ -3403,9 +3403,10 @@ impl ExtensionManager {
 
         // No auth section — setup_is_complete was already checked above,
         // so if we reach here the setup requirements are satisfied.
-        if cap_file.setup.is_none() {
-            return ToolAuthState::NoAuth;
-        }
+        let setup = match &cap_file.setup {
+            Some(s) => s,
+            None => return ToolAuthState::NoAuth,
+        };
 
         let all_provided = futures::future::join_all(
             setup