fix(grpc): preserve forwarded authentication failures

[yordis]

• Forwarded writes must report authentication failures consistently with direct gRPC requests.

[cursor]

PR Summary

Medium Risk
Touches authentication error propagation for gRPC stream writes; behavior change for forwarded auth failures but no change to authorization decisions themselves.

Overview
Forwarded stream Append and Delete now map ClientMessage.NotAuthenticated replies to gRPC Unauthenticated errors (with the server reason), instead of failing as an unknown completion message.

Adds ForwardedAuthenticationTests to assert both RPCs surface StatusCode.Unauthenticated and include "forwarding denied" when the bus replies with NotAuthenticated.

^{Reviewed by Cursor Bugbot for commit 55bef09. Bugbot is set up for automated code reviews on this repo. Configure here.}

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📥 Commits

Reviewing files that changed from the base of the PR and between d30db04 and 55bef09.

📒 Files selected for processing (3)

• src/EventStore.Core.Tests/Services/Transport/Grpc/StreamsTests/ForwardedAuthenticationTests.cs
• src/EventStore.Core/Services/Transport/Grpc/Streams.Append.cs
• src/EventStore.Core/Services/Transport/Grpc/Streams.Delete.cs

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