feat(utilities.text.json): STJ overloads for keystone JsonExtensions + migrate 5 callers (#83)

[ChrisonSimtian]

Summary

Adds System.Text.Json parallel overloads to the JsonExtensions keystone and migrates all 5 internal callers to the STJ surface. Non-breaking on 10.3.x — Newtonsoft surface stays, just [Obsolete] for v11 removal.

JsonExtensions.cs surface

New (STJ):

• DefaultSerializerOptions (System.Text.Json equivalent of DefaultSerializerSettings)
• ToJson<T>(JsonSerializerOptions), GetJson<T>(JsonSerializerOptions)
• GetJsonObject(string) → JsonObject, ReadJsonObject(AbsolutePath) → JsonObject
• WriteJson<T>(AbsolutePath, T, JsonSerializerOptions)
• ReadJson<T>(AbsolutePath, JsonSerializerOptions)
• UpdateJson<T>(AbsolutePath, Action<T>, JsonSerializerOptions)
• UpdateJsonObject(AbsolutePath, Action<JsonObject>)

[Obsolete] for v11 removal (Newtonsoft):

• ToJson<T>(JsonSerializerSettings?, Formatting)
• GetJson<T>(JsonSerializerSettings?), GetJson(JsonSerializerSettings?) → JObject
• WriteJson<T>(JsonSerializerSettings?), ReadJson<T>(JsonSerializerSettings?), ReadJson(JsonSerializerSettings?) → JObject
• UpdateJson<T>(Action<T>, JsonSerializerSettings?), UpdateJson(Action<JObject>, JsonSerializerSettings?)

STJ overloads require an explicit JsonSerializerOptions argument (no default) to avoid overload-resolution ambiguity with the Newtonsoft no-args path. Callers wanting framework defaults pass JsonExtensions.DefaultSerializerOptions.

Migrated callers (5)

File	Pattern
Tools/OctoVersion/OctoVersionTasks.cs	Drops the AllWritableContractResolver workaround (STJ binds records via ctor natively)
Attributes/HandleSingleFileExecutionAttribute.cs	global.json sdk.version lookup uses JsonObject + GetValue<string>
GlobalTool/Program.Secrets.cs	Secret param read/write via JsonObject + Where on the IEnumerable<KeyValuePair<string, JsonNode>> it implements
GlobalTool/Program.Setup.cs	Parameters-file write
GlobalTool/Program.Update.cs	global.json sdk.version write

Drops using Newtonsoft.Json.Linq; from all 5; they're now pure STJ.

Out of scope (still needs Newtonsoft)

• JObject.GetChildren.cs / JObject.GetPropertyValue.cs — still used by the 3 pragma-suppressed call sites from feat(net): STJ overloads for HTTP JSON helpers, [Obsolete] Newtonsoft surface (#83 follow-up) #165 (MastodonTasks, SlackTasks, GitHubActions.Client). Their migration is a separate v11 cutover.
• AllWritableContractResolver, Base64JsonConverter — unused after the OctoVersionTasks migration but kept until the v11 cleanup (external consumers may still reference them).
• Fallout.Tooling, Fallout.Tooling.Generator, Fallout.Build (NJsonSchema swap), the bulk of Fallout.Common runtime + ~60 Tools/*/*.Generated.cs files.

Added System.Text.Json PackageReference to Fallout.Utilities.Text.Json.csproj (STJ isn't in netstandard2.0 BCL). Newtonsoft.Json reference stays for the still-internal Newtonsoft-typed surfaces.

Verification

• dotnet build src/Fallout.Utilities.Text.Json/Fallout.Utilities.Text.Json.csproj — 0 errors
• dotnet build src/Fallout.Common/Fallout.Common.csproj — 0 errors (callers migrated cleanly)
• dotnet build src/Fallout.GlobalTool/Fallout.GlobalTool.csproj — 0 errors
• dotnet build fallout.slnx — 0 errors across the whole solution
• dotnet test tests/Fallout.SourceGenerators.Tests/Fallout.SourceGenerators.Tests.csproj — 6/6 pass

Test plan

• CI green
• nuke :secrets and nuke :update CLI surfaces still work as expected against a real Fallout-using repo
• External consumer using the JsonSerializerSettings overloads sees [Obsolete] warnings pointing to the v11 cutover

🤖 Generated with Claude Code