SignedCms add tests and reduce allocations

[PranavSenthilnathan]

• Add a test verifying that NoSignature works when a cert with an unknown SPKI is supplied
• Add TState to a couple of sites to reduce allocations
• Use stackalloc/arraypool for an allocation

Contributes to #115578

[Copilot]

Pull Request Overview

This PR introduces additional tests for the SignedCms functionality with a particular focus on verifying behavior when a certificate with an unknown SPKI is supplied while also refactoring internal methods to reduce allocations.

• Added tests to validate the NoSignature behavior with an unknown algorithm certificate
• Updated several internal methods in SignerInfo.cs to use generic state parameters and optimized memory usage with stackalloc/arraypool techniques

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Security.Cryptography.Pkcs/tests/SignedCms/SignedCmsTests.cs	Added tests for NoSignature behavior and default digest computation
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/SignerInfo.cs	Updated verify callback methods and optimized allocation using stackalloc and arraypool

Comments suppressed due to low confidence (2)

src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/SignerInfo.cs:680

• The use of the magic number 0x31 here could be clarified by either defining it as a named constant or adding a comment explaining its significance in the encoding process.

encoded[0] = 0x31;

src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/SignerInfo.cs:928

• [nitpick] Consider renaming the tuple element '@this' to a more descriptive name (e.g., 'signerInfo') to improve code clarity in the callback methods.

static (state, contentToVerify) => state.signatureProcessor.VerifySignature(

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[bartonjs]

Suggested change

	(@this: this, signatureProcessor, certificate),
	(info: this, signatureProcessor, certificate),

@this always looks icky.