SearchSettingsPart SearchFields property is overwritten when multiple recipes add search fields

[I3undy]

When defining SearchSettingsPart settings through recipes, the SearchFields property is overwritten instead of merged when multiple recipes attempt to add search fields for different indexes.
This results in losing previously defined search fields if a later recipe sets its own.

Steps to reproduce the behavior

1. Create a recipe that adds the following SearchFields:

<SearchSettingsPart SearchFields="
    Admin:
    UserPart.Email,
    UserPart.FirstName,
    UserPart.LastName" 
    DisplayType="Summary" FilterCulture="false" SearchIndex="" />

2. Create another recipe that adds:

<SearchSettingsPart SearchFields="
    UserSearch:
    UserPart.UserName,
    UserPart.Email,
    UserPart.RegistrationStatus,
    UserPart.CreatedUtc,
    UserPart.LastLoginUtc,
    UserRolesPart.Roles" 
    DisplayType="Summary" FilterCulture="false" SearchIndex="" />

3. Execute both recipes.

Expected behavior
Both Admin and UsersReport search fields should coexist after applying both recipes.

Actual behavior
The last applied recipe overwrites the previous SearchFields settings. Only the fields from the latest recipe are present.

The SearchFields is treated as a single text field and fully replaced rather than merged by the index name. Ideally, the system should merge entries or provide a way to append settings without overwriting previous ones.

[sebastienros]

For reference this is where the string value is converted to a dictionary:

Orchard/src/Orchard.Web/Modules/Orchard.Search/Models/SearchSettingsPart.cs

Line 13 in c38c9bf

var data = SearchSettingsHelper.SerializeSearchFields(value);