Clean up CS8632 warnings in Service.Tests (nullable annotations under <Nullable>disable</Nullable>)

[aaronburtle]

Background

src/Service.Tests/Azure.DataApiBuilder.Service.Tests.csproj declares
<Nullable>disable</Nullable>, but a handful of test files use C# nullable
reference type syntax (e.g. string? result, string? issuer). The compiler
emits CS8632 ("The annotation for nullable reference types should only be
used in code within a '#nullable' annotations context") for every such usage.

Known files (non-exhaustive)

• src/Service.Tests/Configuration/RuntimeConfigLoaderTests.cs (~3 warnings)
• src/Service.Tests/UnitTests/RequestParserUnitTests.cs (~4 warnings)
• src/Service.Tests/UnitTests/SqlQueryExecutorUnitTests.cs (~4 warnings)

A full sweep is needed; the list above came from incidental observation, not
a project-wide audit.

Why it's worth fixing

• The warnings are noise in CI build logs and can mask real issues.
• They are inconsistent with the rest of the repo, which builds clean.
• Some build configurations (e.g. Docker, certain Release flows) already
  enable TreatWarningsAsErrors=True. If Service.Tests ever inherits
  that setting, these become hard build breaks.