Handle Azure AI content safety blocks

[BenjaminMichaelis]

Summary

• Detect Azure Responses API content-filter failures and map them to a generic content_filtered response without returning provider details.
• Return a consistent JSON/SSE moderation error and update the chat UI to display it while removing any partial assistant reply from a blocked stream.
• Add focused tests for provider-error classification and message/stream endpoint contracts.

This is the application-side change for #1065. The paired Terraform change lowers the four harmful-content prompt thresholds for Violence, Hate, Sexual, and Selfharm from High to Low in IntelliTect-dev/EssentialCSharp.AzureResourceManagement.

Validation

• dotnet test EssentialCSharp.Chat.Tests — 32 passed.
• dotnet test EssentialCSharp.Web.Tests — 154 passed (serial run).
• node --check EssentialCSharp.Web/wwwroot/js/chat-module.js — passed.

Azure policy enforcement for the deployed xAI/Grok model still requires environment verification.

Fixes: #1065

[Copilot]

Copilot review overview

🟡 Changes recommended

Non-streaming incomplete content-filter responses can still return partial or empty output with HTTP 200.

Review effort: Balanced
Findings: 2

Open (2)

• Map incomplete content_filter responses to ChatContentFilteredException · New
• Add AIChatService tests for provider error classification · New

What changed in this PR

Adds application handling for Azure AI content-safety blocks across chat services, API endpoints, streaming, and UI behavior.

Changes:

• Classifies provider moderation failures using a domain exception.
• Returns generic JSON/SSE moderation errors and removes blocked partial replies.
• Adds endpoint contract tests and reusable test helpers.

File	Description
EssentialCSharp.Chat.Shared/​Services/​AIChatService.cs	Classifies Azure content-filter failures.
EssentialCSharp.Chat.Shared/​Services/​ChatBackendUnavailableException.cs	Adds the moderation domain exception.
EssentialCSharp.Web/​Controllers/​ChatController.cs	Maps moderation failures to JSON or SSE errors.
EssentialCSharp.Web/​wwwroot/​js/​chat-module.js	Handles moderation events and fragmented SSE data.
EssentialCSharp.Web.Tests/​ChatModerationTests.cs	Tests endpoint moderation contracts.
EssentialCSharp.Web.Tests/​McpTestHelper.cs	Generalizes authentication helpers for derived factories.

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