Migrating away from implementation helpers

The next major version makes the following implementation types internal on both netstandard2.0 and net8.0. Remove direct references to them when upgrading.

Former public type	Supported replacement
BDTest.Helpers.StepTextHelper	Build and execute fluent steps. Use [StepText], WithArgs and WithStepText to control text; read Scenario.Steps afterward. Register custom value converters on BDTestOptions before starting the run.
BDTest.Helpers.StoryTextHelper	Use [Story] and read GetStoryText() on the scenario, test base or context.
BDTest.Helpers.BDTestVersionHelper	Read Version from a report exported with BDTestJsonHelper, or inspect the core assembly's AssemblyInformationalVersionAttribute when no report is needed.
BDTest.Constants.DefaultValues (including ReportServer)	Read resolved story/scenario text from results. Use RunFilter for report query parsing; the historical UI placeholders are handled by the renderer.
BDTest.Output.TimespanConverter, BDTest.Output.StatusConverter	Use BDTestJsonHelper for complete reports, or BDTestJsonContext for generated JSON metadata. Do not register the implementation converters yourself.
BDTest.Helpers.JsonConverters.ScenarioTextConverter, TestInformationAttributeJsonConverter	Use the same supported report serialization entry points.

TypeHelper and StoryTextConverter remain internal. These accessibility changes do not change stored report JSON: historical durations, status values, story and scenario strings, information attributes and legacy report defaults remain readable. BDTestJsonContext remains public and its generated metadata uses the internal converters without requiring reflection-based JSON serialization. For a standalone Status value, use BDTestJsonContext.Default.Status (or copy its converter to your own serializer options); the enum no longer carries an assembly-internal converter attribute. Report status properties retain their converter attributes, so complete reports preserve their wire format even with custom context options. See JSON migration for the historical format and reader.

The unsigned adapters and external consumers receive no friend access to core. Custom adapters should continue using the protected BeginTest, EndTestAsync, CaptureStartupOutput, BindStartupOutput, CompleteStartupOutput, BeginPendingTestExecution and ExecuteScenarioStepsAsync boundary described in custom adapters. The fluent builders retain their concrete return types.

Global BDTestSettings and upload overloads without an explicit run were removed separately. Supply a BDTestRun and BDTestRunDescriptor as shown in explicit run migration; the supported explicit-run upload methods still accept cancellation tokens. Helper accessibility does not replace those methods or move report contracts to a new namespace.