fix(handlers): restore OFFSEC-001 mcp.go scrub after Tier-2e rebase

The rebase of test/settings-tab-coverage onto current main dropped the
duplicate Tier-2e lint files (already in main via PR#689) but
replayed the mcp.go revert from the pre-OFFSEC-001 Tier-2e branch.
Restore the fix:
- mcp.go: constant error message, no req.Method echo
- mcp_test.go: OFFSEC-001 scrub-contract assertions

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/handlers/mcp.go

@@ -434,7 +434,8 @@ func (h *MCPHandler) dispatchRPC(ctx context.Context, workspaceID string, req mc
 		}
 
 	default:
-		base.Error = &mcpRPCError{Code: -32601, Message: "method not found: " + req.Method}
+		// Per OFFSEC-001: error message must not include user-controlled req.Method.
+		base.Error = &mcpRPCError{Code: -32601, Message: "method not found"}
 	}
 
 	return base

workspace-server/internal/handlers/mcp_test.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strings"
 	"testing"
 
 	"errors"
@@ -204,6 +205,9 @@ func TestMCPHandler_NotificationsInitialized_Returns200(t *testing.T) {
 // Unknown method
 // ─────────────────────────────────────────────────────────────────────────────
 
+// TestMCPHandler_UnknownMethod_Returns32601 verifies dispatchRPC returns
+// -32601 for an unknown method. Per OFFSEC-001: the error message must be
+// constant — req.Method is user-controlled and must NOT appear in the response.
 func TestMCPHandler_UnknownMethod_Returns32601(t *testing.T) {
 	h, _ := newMCPHandler(t)
 
@@ -224,6 +228,14 @@ func TestMCPHandler_UnknownMethod_Returns32601(t *testing.T) {
 	if resp.Error.Code != -32601 {
 		t.Errorf("expected code -32601, got %d", resp.Error.Code)
 	}
+	// Message must be constant — no user-controlled method name leak.
+	if resp.Error.Message != "method not found" {
+		t.Errorf("error message should be constant 'method not found', got: %q", resp.Error.Message)
+	}
+	// Double-check the method name never appears in the message (defence-in-depth).
+	if strings.Contains(resp.Error.Message, "not/a/real/method") {
+		t.Error("error message must not echo the user-controlled method name")
+	}
 }
 
 // ─────────────────────────────────────────────────────────────────────────────