feat(justdummies): guard public/internal arguments against null

[Reefact]

Summary

Every public/internal member of JustDummies now rejects a null non-nullable reference-type argument with an ArgumentNullException naming the parameter, so a null fails fast at the boundary instead of surfacing later as a NullReferenceException. A reflection-driven convention test enforces this across the whole surface and holds new members to it automatically.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Analyzer / diagnostic change
• Tests
• Documentation

Changes

• Add null-argument guards (if (x is null) { throw new ArgumentNullException(nameof(x)); }) to every public/internal constructor and method taking a non-nullable reference-type parameter, across all of JustDummies.
• Scope respected: private members are trusted (their inputs are guarded at the public/internal boundary they enter through, e.g. the Create(RandomSource) factories); value-type and nullable (?) parameters are left as-is (a ? parameter's contract permits null).
• Exempt exception-type constructors: they run on the error-handling/logging path, where throwing over a null message would mask the original failure.
• Any.ReproduciblyAsync now validates body synchronously instead of deferring the guard into the returned task.
• Add NullArgumentGuardConventionTests: a reflection test that enumerates the whole public/internal surface and asserts each non-nullable reference parameter throws ArgumentNullException; it covers new members automatically and fails loudly on any parameter it cannot exercise. Excluded from the net472 floor (it uses NullabilityInfoContext, a .NET 6+ API); the guards themselves remain netstandard2.0.
• Open the library's internals to the contract suite (InternalsVisibleTo) so the convention can verify the internal boundary the public API can never route a null through.
• Record the decision as ADR-0045 (EN + FR), indexed.

Testing

• dotnet build FirstClassErrors.sln — 0 warnings / 0 errors.
• dotnet test FirstClassErrors.sln — not run in full; the suites below were run instead.
• Analyzer tests pass (FirstClassErrors.Analyzers.UnitTests) — not run.

Suites actually run (all green):

• dotnet test JustDummies.UnitTests — 556 passed (including the new convention test).
• dotnet test JustDummies.PropertyTests — 220 passed.
• dotnet test FirstClassErrors.Testing.UnitTests — 8 passed (consumer via ADR-0026).
• dotnet test JustDummies.Xunit.UnitTests — 14 passed.
• net472 support-floor build, Release, EnableNet472Floor=true (JustDummies.UnitTests + JustDummies.PropertyTests) — 0 / 0.

Documentation

• Public API / error documentation updated — no public API change (signatures unchanged; async is an implementation detail).
• README / doc/ updated — ADR-0045 added (EN + FR) and indexed.
• French translation (doc/handwritten/for-users/README.fr.md) — not needed (no user-facing README behavior change).
• No documentation change required

Architecture decisions

• No architectural decision in this pull request
• New decision recorded — ADR-0045 (accepted by the maintainer in-session; opens InternalsVisibleTo, deliberately relaxing the previously black-box test posture recorded in the 2026-07-20 audit §9.3).

Related issues

None.

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https://claude.ai/code/session_01KR3opZZYgSiomTKWvC11q8

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