ILTrim: make custom-attribute type-name rewrite best-effort

[Copilot]

This stacked PR updates the custom-attribute value rewrite path in ILTrim to avoid failing on unresolved serialized type names. Rewriting now happens only when type-name resolution succeeds, preserving original payload data otherwise.

• ## Description
  • Non-throwing resolution in rewrite path
    • Uses non-throwing custom-attribute type-name resolution when parsing serialized type names.
  • Conditional rewrite
    • Replaces the serialized type-name string only when resolution returns a concrete type.
  • Preserve unresolved values
    • Leaves unresolved serialized type names unchanged instead of throwing during blob rewrite.

TypeDesc? resolved =
    _factory.TypeSystemContext.GetTypeByCustomAttributeTypeName(s, _module, throwIfNotFound: false);

if (resolved is not null)
{
    s = TypeNameFormatter.FormatNameForReflection(resolved);
}

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