[Repo Assist] Add JSON SaveOptions property tests + STJ benchmark prototype (#1671)

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Task 9 — Testing Improvements: JSON property tests for all SaveOptions

Extends JsonParserProperties.fs with two new FsCheck property tests that were previously missing:

• Parsing JsonValue formatted with None (indented) returns the same JsonValue — 500 random JsonValue trees serialized with JsonSaveOptions.None (pretty-printed/indented) and parsed back; verifies the formatted output is valid JSON that round-trips correctly.
• Parsing JsonValue formatted with CompactSpaceAfterComma returns the same JsonValue — same check for JsonSaveOptions.CompactSpaceAfterComma (introduced in A new FSharp.Data.JsonSaveOptions value needed #1482).

The existing DisableFormatting property test (1000 cases) is unchanged. All three save-option variants are now covered by property tests.

Task 10 — Take the Repository Forward: STJ benchmark prototype for #1671

Adds tests/FSharp.Data.Benchmarks/JsonStjBenchmarks.fs — a working prototype of "Option 2" from the #1671 discussion (keep the JsonValue public API, use System.Text.Json as the parsing kernel).

The StjConverter module demonstrates the conversion path:

string → JsonDocument.Parse → walk JsonElement → JsonValue

JsonStjBenchmarks benchmarks this against the current hand-written parser on three real-world JSON files (GitHub issues, Twitter sample, WorldBank). Running dotnet run -c Release -- stj will give a direct comparison of:

• Parse throughput (ops/sec)
• Allocations (bytes/op)

This gives concrete data for the #1671 discussion without requiring any source-code changes to the main library. The StjConverter prototype lives entirely in the benchmarks project.

No new NuGet dependencies — System.Text.Json is part of the net8.0 shared framework (the benchmarks project already targets net8.0).

Also updates Program.fs to accept stj as a command-line argument and includes JsonStjBenchmarks in the default "run all" path.

Test Status

• ✅ Build: 0 errors, 6 pre-existing warnings (NETSDK1212 trim warnings, pre-existing FS1182)
• ✅ All 7 JsonParserProperties tests pass (including the 2 new property tests)
• ✅ Benchmarks project builds cleanly

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