[STJ-2] Migrate isolated JObject.Parse / JsonConvert callsites (no-utility-lib group) #115

Sub-task of #83 (Newtonsoft.Json → System.Text.Json migration).

Scope

Migrate JObject.Parse(...) / JsonConvert.DeserializeObject(...) callsites that don't go through Fallout.Utilities.Text.Json. These can be migrated independently of the utility-lib rewrite.

Already done

• src/Fallout.SourceGenerators/StronglyTypedSolutionGenerator.cs — landed on main in 54a1655.

Remaining files (likely candidates — confirm during the PR)

• src/Fallout.Build/Execution/Extensions/ArgumentsFromParametersFileAttribute.cs — JObject.Parse(File.ReadAllText(parametersFile)) + .Property(name).Value.ToObject<T>() patterns
• src/Fallout.Common/CI/GitHubActions/GitHubActions.Client.cs — uses utility lib (BLOCKED on STJ-3)
• src/Fallout.Common/CI/GitHubActions/GitHubActions.cs — uses utility lib (BLOCKED on STJ-3)
• src/Fallout.Common/Attributes/HandleSingleFileExecutionAttribute.cs — check
• src/Fallout.Utilities.Net/HttpRequest.Content.cs + HttpResponse.Body.cs — check (might be utility-lib-coupled)

Any file that uses our Fallout.Utilities.Text.Json extension methods (ReadJson, WriteJson, JObjectExtensions.GetPropertyValue, etc.) needs to wait for STJ-3.

Acceptance

• All listed files that don't go through the utility lib are migrated
• No new Newtonsoft references introduced
• ./build.ps1 Test green