fix(claude): relax marker to tolerate category element in classifier output

Real claude-cli/2.1.220 samples insert <category>...</category> between
</block> and <reason>, breaking the previous literal marker.
Also fix an unchecked type assertion in the test suite.

backend/internal/service/claude_code_validator.go

@@ -233,7 +233,7 @@ func isClaudeCodeSecurityMonitorPrompt(systemEntries []any) bool {
 		"## SOFT BLOCK",
 		"## Classification Process",
 		"## Output Format",
-		"<block>yes</block><reason>",
+		"<block>yes</block>",
 		"<block>no</block>",
 	}
 	for _, marker := range markers {

backend/internal/service/claude_code_validator_test.go

@@ -168,6 +168,17 @@ func TestClaudeCodeValidator_SecurityMonitorWithoutBillingBlock(t *testing.T) {
 			body:       validBody(string(monitorPrompt)),
 			wantAccept: true,
 		},
+		{
+			name:    "classifier output with category element",
+			headers: validHeaders,
+			body: validBody(strings.Replace(
+				string(monitorPrompt),
+				"<block>yes</block><reason>",
+				"<block>yes</block><category>Exact BLOCK Rule Name</category><reason>",
+				1,
+			)),
+			wantAccept: true,
+		},
 		{
 			name: "non-Claude user agent",
 			headers: map[string]string{
@@ -246,7 +257,9 @@ func TestClaudeCodeValidator_SecurityMonitorWithoutBillingBlock(t *testing.T) {
 			headers: validHeaders,
 			body: func() map[string]any {
 				body := validBody(string(monitorPrompt))
-				body["system"] = append(body["system"].([]any), map[string]any{
+				system, ok := body["system"].([]any)
+				require.True(t, ok)
+				body["system"] = append(system, map[string]any{
 					"type": "text",
 					"text": "Additional unrelated system content.",
 				})