test(security): use valid X.509 PEM certificates in buildSecurityContext tests

Author: ZhuchkaTriplesix

test/core/security/ssl_certificate_support_test.dart

@@ -116,12 +116,61 @@ void main() {
       expect(ctx, isNull);
     });
 
+    const validCertPem = '''-----BEGIN CERTIFICATE-----
+MIIDCTCCAfGgAwIBAgIUaf9MPYX80yFJlevK8mydpFPRUSUwDQYJKoZIhvcNAQEL
+BQAwFDESMBAGA1UEAwwJbG9jYWxob3N0MB4XDTI2MDkyNTA3MjQzMVoXDTI3MDky
+NTA3MjQzMVowFDESMBAGA1UEAwwJbG9jYWxob3N0MIIBIjANBgkqhkiG9w0BAQEF
+AAOCAQ8AMIIBCgKCAQEAsqAeJVJ6+kJnPFp9Bzjm8pjxTLOSJ0DvkJ+BsHF/rW0H
+hDU5rFY6xgMUyM4Y2bXBa+BVSpnhzVEr6zOr5hebDq7fbTrtL3YSiCtxEVqlqjZg
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+3dcW4xSuDp00JS6xLg==
+-----END CERTIFICATE-----''';
+
+    const validKeyPem = '''-----BEGIN PRIVATE KEY-----
+MIIEvAIBADANBgkqhkiG9w0BAQEFAASCBKYwggSiAgEAAoIBAQCyoB4lUnr6Qmc8
+Wn0HOObymPFMs5InQO+Qn4GwcX+tbQeENTmsVjrGAxTIzhjZtcFr4FVKmeHNUSvr
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+/9Jl2lm7gNBC/JUSrwoL7hyBwWgXZZWFF8YkDwyZwNoRcCS6ZRy3J7AlomlFEwVu
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+686S5vSkXmZ67rUTaCGLHFJGIhG+VPz6h3S5StEdf4I9PLUZaMDzFZDo4ZkEyR56
+DQGrf1O526GeqzmO5XVoX572Iuc67DcR8jAKkLECgYAkPigDPHZ5d5kFcS/YEya2
+Rew/doODqiEstN7GO89KNLi1dDbuUOljs8ii2WY71XE7KQFBN2Fv4qVxdl3AfgnJ
+kbu2ounmhZaVEbvHOw1zCcCYo25vBxcAa8zoBX2z2CAfN7/vkdOHKVDPr+wSnwaS
+eQ6/4mA+Eqf0w+s+IOflBw==
+-----END PRIVATE KEY-----''';
+
     test('buildSecurityContext initializes SecurityContext with system trusted roots', () async {
       final testDir = await Directory.systemTemp.createTemp('querya_sec_ctx_');
       final certFile = File('${testDir.path}/client.crt');
       final keyFile = File('${testDir.path}/client.key');
-      await certFile.writeAsString('-----BEGIN CERTIFICATE-----\nTEST_CERT\n-----END CERTIFICATE-----\n');
-      await keyFile.writeAsString('-----BEGIN PRIVATE KEY-----\nTEST_KEY\n-----END PRIVATE KEY-----\n');
+      await certFile.writeAsString(validCertPem);
+      await keyFile.writeAsString(validKeyPem);
 
       final paths = SslCertificatePaths(
         clientCert: certFile.path,
@@ -136,7 +185,7 @@ void main() {
     test('buildSecurityContext applies custom root CA certificate', () async {
       final testDir = await Directory.systemTemp.createTemp('querya_sec_ctx_root_');
       final rootFile = File('${testDir.path}/root.crt');
-      await rootFile.writeAsString('-----BEGIN CERTIFICATE-----\nROOT_CA\n-----END CERTIFICATE-----\n');
+      await rootFile.writeAsString(validCertPem);
 
       final paths = SslCertificatePaths(
         rootCert: rootFile.path,