.NET: Fix OpenAIResponsesAgentClient to include agentName in endpoint path (#5748)

* Fix OpenAIResponsesAgentClient endpoint to include agentName in path (#5324)

The sample OpenAIResponsesAgentClient used '/v1/' as the endpoint, which
routes to the multi-agent endpoint requiring agent.name in the request body.
However, AsIChatClient(agentName) maps agentName to the model field, not
agent.name, causing HTTP 400 errors on OpenAI-compatible endpoints.

Changed the endpoint to '/{agentName}/v1/' to match the pattern used by
OpenAIChatCompletionsAgentClient, routing to the single-agent endpoint
where no agent.name body field is needed.

Added regression test verifying that the model field alone is insufficient
for agent resolution on the multi-agent endpoint.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review feedback for #5324

- URL-escape agentName in OpenAIResponsesAgentClient endpoint path to
  handle reserved characters safely
- Add per-agent MapOpenAIResponses() calls in AgentHost so the sample
  host serves the /{agentName}/v1/responses routes the client now targets
- Replace brittle Assert.Contains("agent.name") assertions with stable
  machine-readable error code assertion ("missing_required_parameter")

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address additional review feedback for #5324

- Apply Uri.EscapeDataString to OpenAIChatCompletionsAgentClient endpoint
  for consistency with OpenAIResponsesAgentClient
- Map OpenAI Responses and ChatCompletions endpoints for all builder-based
  agents (chemist, mathematician, literator, science workflows) so every
  discoverable agent is reachable via the single-agent endpoint path

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/samples/05-end-to-end/AgentWebChat/AgentWebChat.AgentHost/Program.cs

@@ -163,10 +163,22 @@ app.MapA2AHttpJson(knightsKnavesAgentBuilder, path: "/a2a/knights-and-knaves");
 app.MapDevUI();
 
 app.MapOpenAIResponses();
+app.MapOpenAIResponses(pirateAgentBuilder);
+app.MapOpenAIResponses(knightsKnavesAgentBuilder);
+app.MapOpenAIResponses(chemistryAgent);
+app.MapOpenAIResponses(mathsAgent);
+app.MapOpenAIResponses(literatureAgent);
+app.MapOpenAIResponses(scienceSequentialWorkflow);
+app.MapOpenAIResponses(scienceConcurrentWorkflow);
 app.MapOpenAIConversations();
 
 app.MapOpenAIChatCompletions(pirateAgentBuilder);
 app.MapOpenAIChatCompletions(knightsKnavesAgentBuilder);
+app.MapOpenAIChatCompletions(chemistryAgent);
+app.MapOpenAIChatCompletions(mathsAgent);
+app.MapOpenAIChatCompletions(literatureAgent);
+app.MapOpenAIChatCompletions(scienceSequentialWorkflow);
+app.MapOpenAIChatCompletions(scienceConcurrentWorkflow);
 
 // Map the agents HTTP endpoints
 app.MapAgentDiscovery("/agents");

dotnet/samples/05-end-to-end/AgentWebChat/AgentWebChat.Web/OpenAIChatCompletionsAgentClient.cs

@@ -24,7 +24,7 @@ internal sealed class OpenAIChatCompletionsAgentClient(HttpClient httpClient) :
     {
         OpenAIClientOptions options = new()
         {
-            Endpoint = new Uri(httpClient.BaseAddress!, $"/{agentName}/v1/"),
+            Endpoint = new Uri(httpClient.BaseAddress!, $"/{Uri.EscapeDataString(agentName)}/v1/"),
             Transport = new HttpClientPipelineTransport(httpClient)
         };
 

dotnet/samples/05-end-to-end/AgentWebChat/AgentWebChat.Web/OpenAIResponsesAgentClient.cs

@@ -23,7 +23,7 @@ internal sealed class OpenAIResponsesAgentClient(HttpClient httpClient) : AgentC
     {
         OpenAIClientOptions options = new()
         {
-            Endpoint = new Uri(httpClient.BaseAddress!, "/v1/"),
+            Endpoint = new Uri(httpClient.BaseAddress!, $"/{Uri.EscapeDataString(agentName)}/v1/"),
             Transport = new HttpClientPipelineTransport(httpClient)
         };