Remove JObject ServiceIndex

[Nigusu-Allehu]

Bug

Fixes: NuGet/Home#14913

Description

Migrates GetServiceIndexResponse.ServiceIndex and GetOperationClaimsRequest.ServiceIndex
from JObject to string in NuGet.Protocol.Plugins.

The migration follows the same breaking change pattern as #7376: the old JObject constructors and
properties are marked [Obsolete] and delegate to the new string-based API. A custom NSJ
converter (NsjRawJsonStringConverter) enforces that only JSON objects are accepted on both read
and write.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET SDK Breaking Change Notification email list.

You can refer to the .NET SDK breaking change guidelines

[Copilot]

Pull request overview

This PR updates the NuGet protocol plugin message surface to stop exposing JObject for service index payloads, replacing it with a raw JSON string representation while keeping the on-the-wire JSON shape as an object.

Changes:

• Introduces ServiceIndexJson (string) on GetServiceIndexResponse and GetOperationClaimsRequest, and obsoletes the JObject APIs.
• Adds a custom Newtonsoft.Json converter (NsjRawJsonStringConverter) intended to enforce “JSON object only” semantics for these raw JSON string properties.
• Updates unit/functional tests and PublicAPI.Unshipped entries to reflect the new API surface.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/NuGet.Core.Tests/NuGet.Protocol.Tests/Plugins/RequestHandlers/GetServiceIndexRequestHandlerTests.cs	Updates assertions to validate ServiceIndexJson instead of JObject formatting.
test/NuGet.Core.Tests/NuGet.Protocol.Tests/Plugins/NsjRawJsonStringConverterTests.cs	Adds focused tests for the new raw-JSON-string converter.
test/NuGet.Core.Tests/NuGet.Protocol.Tests/Plugins/Messages/GetServiceIndexResponseTests.cs	Migrates message tests to ServiceIndexJson and validates JSON shape.
test/NuGet.Core.Tests/NuGet.Protocol.Tests/Plugins/Messages/GetOperationClaimsRequestTests.cs	Migrates message tests to ServiceIndexJson and validates JSON shape.
test/NuGet.Core.FuncTests/NuGet.Protocol.FuncTest/PluginTests.cs	Updates functional tests to pass service index as a raw JSON string.
src/NuGet.Core/NuGet.Protocol/PublicAPI/net8.0/PublicAPI.Unshipped.txt	Adds Unshipped entries for the new string-based APIs.
src/NuGet.Core/NuGet.Protocol/PublicAPI/net472/PublicAPI.Unshipped.txt	Adds Unshipped entries for the new string-based APIs.
src/NuGet.Core/NuGet.Protocol/Plugins/RequestHandlers/GetServiceIndexRequestHandler.cs	Stops parsing service index into JObject; returns raw JSON string through the new API.
src/NuGet.Core/NuGet.Protocol/Plugins/PluginManager.cs	Stops parsing service index into JObject; propagates raw JSON string through operation-claims flow.
src/NuGet.Core/NuGet.Protocol/Plugins/NsjRawJsonStringConverter.cs	New Newtonsoft.Json converter for treating raw JSON object values as strings.
src/NuGet.Core/NuGet.Protocol/Plugins/Messages/GetServiceIndexResponse.cs	Adds ServiceIndexJson and obsoletes JObject members.
src/NuGet.Core/NuGet.Protocol/Plugins/Messages/GetOperationClaimsRequest.cs	Adds ServiceIndexJson and obsoletes JObject members.
src/NuGet.Core/NuGet.Protocol/GlobalSuppressions.cs	Updates suppressions for the adjusted PluginManager signatures.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (2)

src/NuGet.Core/NuGet.Protocol/Plugins/Messages/GetServiceIndexResponse.cs:52

• Adding the new string overload alongside the existing JObject constructor makes common calls like new GetServiceIndexResponse(MessageResponseCode.NotFound, null) ambiguous at compile time (null can convert to both string and JObject). This is a source-breaking change compared to the previous API and also forces internal callers to add casts. Consider adding a dedicated overload/factory for the no-service-index cases (e.g., a ctor that omits the serviceIndex parameter for NotFound/non-Success scenarios) so callers don’t need to pass null to disambiguate overloads.

        [JsonConstructor]
        public GetServiceIndexResponse(MessageResponseCode responseCode, string serviceIndex)
        {
            if (!Enum.IsDefined(typeof(MessageResponseCode), responseCode))
            {

src/NuGet.Core/NuGet.Protocol/Plugins/Messages/GetOperationClaimsRequest.cs:46

• The new (string, string) overload combined with the existing (string, JObject) constructor makes new GetOperationClaimsRequest(packageSourceRepository, null) ambiguous at compile time (null is convertible to both reference types). Since the remarks explicitly allow null serviceIndex/packageSourceRepository for source-agnostic requests, consider adding a dedicated overload/factory for the source-agnostic/null cases so callers don’t need casts to select the intended overload.

        /// <param name="serviceIndex">The service index (index.json) as a raw JSON string.</param>
        /// <remarks>Both packageSourceRepository and service index can be null. If they are, the operation claims request is considered as source agnostic</remarks>
        [JsonConstructor]
        public GetOperationClaimsRequest(string packageSourceRepository, string serviceIndex)
        {
            PackageSourceRepository = packageSourceRepository;