Fix PR review issues: retry off-by-one, URI encoding, docs, tests, build

- Fix off-by-one in FoundryToolboxBearerTokenHandler retry loop (4 attempts → 3)
- URI-encode version parameter in HostedMcpToolboxAITool.BuildAddress
- Add XML doc clarifying version pinning is reserved for future use
- Add comment clarifying AddHostedService deduplication safety
- Fix DevTemporaryTokenCredential expiry to use DateTimeOffset.MaxValue
- Fix AgentCard ambiguity in A2AServer sample with using alias
- Add 18 new unit tests for retry handler and ReadMcpToolboxMarkers

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

dotnet/samples/04-hosting/FoundryHostedAgents/responses/Hosted-Toolbox/Program.cs

@@ -108,6 +108,6 @@ internal sealed class DevTemporaryTokenCredential : TokenCredential
             throw new CredentialUnavailableException($"{EnvironmentVariable} environment variable is not set.");
         }
 
-        return new AccessToken(this._token, DateTimeOffset.UtcNow.AddHours(1));
+        return new AccessToken(this._token, DateTimeOffset.MaxValue);
     }
 }

dotnet/samples/05-end-to-end/A2AClientServer/A2AServer/HostAgentFactory.cs

@@ -8,6 +8,7 @@ using Microsoft.Agents.AI;
 using Microsoft.Extensions.AI;
 using OpenAI;
 using OpenAI.Chat;
+using AgentCard = A2A.AgentCard;
 
 namespace A2AServer;