fix(handlers): OFFSEC-001 — scrub req.Method from dispatchRPC default error

Line 443 of mcp.go concatenated user-controlled req.Method into the
JSON-RPC -32601 error message, allowing an agent or canvas client to
inject arbitrary strings into the response via the method field.

Fix: replace "method not found: " + req.Method with the constant
"method not found" — matching the OFFSEC-001 scrub contract applied
to the InvalidParams (line 428) and UnknownTool (line 433) paths.

Test: extend TestMCPHandler_UnknownMethod_Returns32601 with two new
assertions:
  1. resp.Error.Message == "method not found"
  2. defence-in-depth check that the sent method name never appears
     in the response (strings.Contains guard)

Issue: #684

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

workspace-server/internal/handlers/mcp.go

@@ -440,7 +440,8 @@ 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")
+	}
 }
 
 // ─────────────────────────────────────────────────────────────────────────────