* .Net: Add hosted agent observability sample
Mirrors the Python sample added in #5608 for Foundry hosted agents. The
.NET hosting library already wires OpenTelemetry automatically via
Microsoft.Agents.AI.Foundry.Hosting (ApplyOpenTelemetry) plus
Azure.AI.AgentServer.Core's AddAgentHostTelemetry, so no framework
changes are needed. The sample is documentation plus a runnable artifact
that produces an interesting span tree (invoke_agent / agent_invoke /
chat / execute_tool).
Adds Hosted-Observability under FoundryHostedAgents/responses with two
small tools (GetCurrentLocation, GetWeather), agent.yaml /
agent.manifest.yaml declaring OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT
(the .NET equivalent of Python's ENABLE_SENSITIVE_DATA), Dockerfile +
Dockerfile.contributor, .env.example and README explaining the .NET vs
Python defaults. Project added to agent-framework-dotnet.slnx.
* Address PR feedback: use Random.Shared and add .dockerignore
Roger Barreto