Harden webhook ingestion against outsiders - #2097

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-dotnet@v4, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Dereference of a possibly null reference.

'IMiscellaneousClient.RenderArbitraryMarkdown(NewArbitraryMarkdown)' is obsolete: 'This client is being deprecated and will be removed in the future. Use MarkdownClient.RenderArbitraryMarkdown instead.'

Dereference of a possibly null reference.

Non-nullable property 'RecomputedHash' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Non-nullable property 'StoredHash' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Nullability of reference types in value of type 'Dictionary<Guid, (string PayloadJson, string? OutcomeJson)>' doesn't match target type 'IReadOnlyDictionary<Guid, (string? PayloadJson, string? OutcomeJson)>'.

Possible null reference assignment.

Possible null reference assignment.

Possible null reference assignment.

The type 'TRequest' cannot be used as type parameter 'TRequest' in the generic type or method 'IPipelineBehavior<TRequest, TResponse>'. Nullability of type argument 'TRequest' doesn't match 'notnull' constraint.