Fix regression introduced in #123925

[jgh07]

#123925 breaks the functionality introduced in #119935.

This PR fixes this issue and adds an appropriate test. I decided to also address the comments in #124537 here.

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[Copilot]

Pull request overview

Fixes a regression in PersistedAssemblyBuilder signature emission that broke persisting function-pointer signatures with custom modifiers (modreq/modopt), and adds a regression test to cover the scenario.

Changes:

• Ensure function-pointer return/parameter custom modifiers are written when encoding function-pointer signatures.
• Extend SaveFunctionPointerFields coverage to include an unmanaged function pointer with multiple calling conventions (including SuppressGCTransition).
• Update test utility helper nullability annotations for modifier/calling-convention arrays.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Reflection.Emit/tests/Utilities.cs	Updates helper types used by tests to accept nullable arrays for conventions/modifiers.
src/libraries/System.Reflection.Emit/tests/PersistedAssemblyBuilder/AssemblySaveTypeBuilderTests.cs	Adds a new regression assertion via a delegate* unmanaged[Fastcall, SuppressGCTransition]<void> field persisted to disk and reloaded.
src/libraries/System.Reflection.Emit/src/System/Reflection/Emit/SignatureHelper.cs	Writes required/optional custom modifiers for function-pointer return/parameter types during signature encoding.

[jgh07]

Actually, it looks like this is only an issue in #123819, I'll add this commit there instead.