Fix #4491: route Target._random through Random.Shared

[jeremydmiller]

Summary

Closes #4491.

src/Marten.Testing/Documents/Target.cs declared _random as a static new Random(67) shared across the test assembly. Random is not thread-safe; xUnit parallelizes across test classes, so concurrent calls into Random.Next(int, int) could corrupt the internal state and start returning 0 indefinitely. That mapped every Target.Color to the 0 arm of the switch (Blue) and made tests that rely on a color distribution fail intermittently:

Shouldly.ShouldAssertException : greens
    should not be empty but was

Surfaced on the PGLatest + STJ + NET10 CI matrix (the fastest of the four, so widest concurrency overlap) on PRs #4488 and #4490 for DocumentDbTests.Reading.query_plans.use_as_batch. Re-running the matrix passed cleanly both times — classic narrow concurrency-window flake.

Fix

Switch the default source to System.Random.Shared (per-thread, thread-safe, .NET 6+). Preserve the existing ResetRandomSeed(int) contract via a [ThreadStatic] overlay: tests that pin a specific seed get a per-thread Random seeded to that value; other code falls back to Random.Shared. The overlay also keeps seeding from leaking across xUnit-parallelized test classes the way the old shared-static field did.

System.Random.Shared is fully qualified at the call site because Target itself has a static Random() factory method that shadows the type name.

Tradeoff

This removes the (already-fragile) process-wide deterministic stream the old static seed implied. The previous comment block on the static field acknowledged that the "determinism" depended on xUnit test discovery order and was a known source of CI flakes — i.e. it was deterministic in name only. Random.Shared trades that for true thread-safety, which matters more in practice.

ResetRandomSeed has no callers in the current codebase, but the method is preserved for source compatibility. Tests that want a deterministic stream still get one on their own thread.

Test plan

• dotnet build src/Marten.slnx -c Release — 0 errors
• DocumentDbTests.Reading.query_plans.* — 2/2 pass on both Newtonsoft and STJ
• DocumentDbTests.Reading.* — 126/126 pass on net10.0
• No ResetRandomSeed callers exist in the codebase — the overlay path is exercised only by future seeded-determinism callers

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