Guard crypto XML XSLT tests on dynamic code support

[Copilot]

Cryptography XML XSLT tests fail on tvOS arm64 because XSLT requires dynamic code support, and that platform throws NotSupportedException before the tests reach their expected crypto failure path.

• Test guards
  • Converted the affected XSLT tests from [Fact] to the existing dynamic-code guard:

[ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))]

• Affected tests
  • SignedXml_EncryptedDataWithInfiniteXslTransform
  • EncryptedKey_InfiniteLoopXsltTransform
  • EncryptedXml_BillionLaughsXsltTransform

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[Copilot]

Pull request overview

This PR updates XSLT-related System.Security.Cryptography.Xml tests to skip on platforms without dynamic code support, matching the capability required by XslCompiledTransform before these tests can reach their intended cryptographic failure assertions.

Changes:

• Replaces [Fact] with [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))] for three XSLT security tests.
• Keeps the existing expected CryptographicException coverage on platforms where reflection emit/dynamic code is supported.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography.Xml/tests/SignedXmlTest.cs	Guards the signed XML encrypted-data XSLT test on reflection emit support.
src/libraries/System.Security.Cryptography.Xml/tests/EncryptedXmlTests.cs	Guards two encrypted XML XSLT transform tests on reflection emit support.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0