.NET: Hosted Agents - RAG Sample with Azure AI Search (#5693) (#5701)

* .NET: Hosted Agents - RAG Sample with Azure AI Search (#5693)

Adds a Hosted-AzureSearchRag sample plus a live Foundry.Hosting integration
test scenario backed by a real Azure AI Search index.

Sample (Hosted-AzureSearchRag): keyword-only Azure AI Search via
SearchClient adapter into TextSearchProvider, scope-aware
DevTemporaryTokenCredential consuming AZURE_BEARER_TOKEN_FOUNDRY +
AZURE_BEARER_TOKEN_SEARCH for local Docker, Dockerfile + contributor
Dockerfile mirroring Hosted-TextRag.

Integration test: AzureSearchRagHostedAgentFixture extends the PR #5598
HostedAgentFixture with the new azure-search-rag scenario branch in the
shared test container; AzureSearchRagHostedAgentTests asserts the model
returns canary tokens (TR-CANARY-7821, SHIP-CANARY-4493) that exist only
in the seeded documents - real proof the agent grounded its answer in
retrieved content rather than training data.

* Address PR 5701 Copilot review feedback

- Sample README: drop stale 'bootstraps the index on first run' line; index is pre-provisioned out of band

- Sample + TestContainer search adapters: propagate CancellationToken to await foreach via .WithCancellation()

.github/workflows/dotnet-build-and-test.yml

@@ -60,6 +60,7 @@ jobs:
               - 'dotnet/src/Microsoft.Agents.AI.Workflows/**'
               - 'dotnet/tests/Foundry.Hosting.IntegrationTests/**'
               - 'dotnet/tests/Foundry.Hosting.IntegrationTests.TestContainer/**'
+              - 'dotnet/samples/04-hosting/FoundryHostedAgents/responses/Hosted-AzureSearchRag/**'
               - 'dotnet/Directory.Packages.props'
               - 'dotnet/tests/Foundry.Hosting.IntegrationTests/scripts/it-build-image.ps1'
               - '.github/workflows/dotnet-build-and-test.yml'
@@ -407,6 +408,12 @@ jobs:
         env:
           AZURE_AI_PROJECT_ENDPOINT: ${{ vars.IT_HOSTED_AGENT_PROJECT_ENDPOINT }}
           AZURE_AI_MODEL_DEPLOYMENT_NAME: ${{ vars.IT_HOSTED_AGENT_MODEL_DEPLOYMENT_NAME }}
+          # Azure AI Search (for the azure-search-rag scenario). Reuses the integration
+          # environment secrets shared with python-sample-validation.yml. The index is
+          # provisioned out of band; see dotnet/tests/Foundry.Hosting.IntegrationTests/README.md
+          # for the required schema and seed content.
+          AZURE_SEARCH_ENDPOINT: ${{ secrets.AZURE_SEARCH_ENDPOINT }}
+          AZURE_SEARCH_INDEX_NAME: ${{ secrets.AZURE_SEARCH_INDEX_NAME }}
           # IT_HOSTED_AGENT_IMAGE was exported into $GITHUB_ENV by the previous step.
 
   # This final job is required to satisfy the merge queue. It must only run (or succeed) if no tests failed