feat!: close the #83 Newtonsoft migration tree — STJ-6 + STJ-2 tail (#119, #115)

[ChrisonSimtian]

Summary

Last-mile Newtonsoft removal. After this lands, no Fallout source file uses Newtonsoft.Json — the .dll survives in the closure only as a transitive of NuGet.Packaging (Fallout.Tooling) and Serilog.Formatting.Compact.Reader (Fallout.Build). The Newtonsoft.Json PackageVersion is removed from Directory.Packages.props entirely.

Closes the entire #83 STJ-migration tree:

• ✓ STJ-1 ([STJ-1] Replace NJsonSchema.NewtonsoftJson with NJsonSchema core (STJ) #114) — SchemaUtility rewritten on STJ
• ✓ STJ-2 ([STJ-2] Migrate isolated JObject.Parse / JsonConvert callsites (no-utility-lib group) #115) — isolated callsites (last one: ArgumentsFromParametersFileAttribute)
• ✓ STJ-3 ([STJ-3] BREAKING: rewrite Fallout.Utilities.Text.Json on System.Text.Json #116) — Fallout.Utilities.Text.Json Newtonsoft surface removed
• ✓ STJ-4 ([STJ-4] Migrate Fallout.Tooling JSON serialization (blocked on STJ-3) #117) — Fallout.Tooling engine on STJ
• ✓ STJ-5 ([STJ-5] Migrate Fallout.Tooling.Generator JSON model (blocked on STJ-3) #118) — Fallout.Tooling.Generator on STJ
• ✓ STJ-6 ([STJ-6] Migrate Fallout.Common.Tools.*Tasks API integrations #119) — *Tasks.cs API integrations on STJ ← this PR
• ✓ Migrate from Newtonsoft.Json to System.Text.Json #83 parent tracker

Migrations in this PR

File	Change
Fallout.Common/Tools/Slack/SlackTasks.cs	JsonConvert.SerializeObject → JsonSerializer.Serialize. [JsonProperty] → [JsonPropertyName] on SlackMessageActionButton.Type.
Fallout.Common/Tools/Teams/Teams.Extensions.cs	[JsonProperty(\"@type\")] / [JsonProperty(\"@context\")] → [JsonPropertyName(...)].
Fallout.Common/Tools/Twitter/TwitterTasks.cs	Internal error-body parsing swapped from JObject.Parse to JsonNode.Parse.
Fallout.Common/CI/GitHubActions/GitHubActions.cs	Lazy<JObject> _eventContext → Lazy<JsonObject>. Public GitHubEvent property type changes from JObject → JsonObject (v11 API break — was tracked in code comment).
Fallout.Build/Execution/Extensions/ArgumentsFromParametersFileAttribute.cs (#115 tail)	parameters.json reading: JObject.Parse + JProperty + ToObject<T> → JsonNode.Parse + indexer + GetValue<T> / Deserialize.
Fallout.Utilities.Net/HttpRequest.Content.cs	Default WithJsonContent now uses STJ. [Obsolete] JsonSerializerSettings overload removed.
Fallout.Utilities.Net/HttpResponse.Body.cs	Default GetBodyAsJson<T> now uses STJ. All [Obsolete] Newtonsoft overloads removed (including JObject-returning ones). GetBodyAsJsonObject returns JsonObject.

Package refs dropped

• Newtonsoft.Json PackageReference removed from Fallout.Utilities.Net.csproj.
• Newtonsoft.Json PackageVersion removed from Directory.Packages.props.

Closure check

$ grep \"Newtonsoft\" src/Fallout.*/bin/.../*.deps.json
# Survives only as transitive via NuGet.Packaging + Serilog.Formatting.Compact.Reader.

Test plan

• dotnet build fallout.slnx — 0 errors.
• dotnet test fallout.slnx — 425/425 pass across 10 DLLs.
• CI ubuntu-latest green.

Closes

• [STJ-6] Migrate Fallout.Common.Tools.*Tasks API integrations #119 (STJ-6)
• [STJ-2] Migrate isolated JObject.Parse / JsonConvert callsites (no-utility-lib group) #115 (STJ-2 tail)
• Migrate from Newtonsoft.Json to System.Text.Json #83 (parent tracker — entire Newtonsoft → STJ migration complete)

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