Rename @params to parameters and add MCP9003 diagnostic for obsolete constructor

- Rename the `@params` constructor parameter to `parameters` in the 3-arg
  RequestContext<TParams> constructor.
- Add MCP9003 diagnostic ID to Obsoletions.cs for the obsolete 2-arg
  constructor, following the established pattern (Message, DiagnosticId,
  UrlFormat).
- Update the [Obsolete] attribute to use the centralized constants with
  DiagnosticId and UrlFormat.
- Document MCP9003 in docs/list-of-diagnostics.md.

Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>
Agent-Logs-Url: https://github.com/modelcontextprotocol/csharp-sdk/sessions/6e17cd63-6cb0-4d77-ba6c-1f2a616fa03f

Author: Copilot

docs/list-of-diagnostics.md

@@ -36,3 +36,4 @@ When APIs are marked as obsolete, a diagnostic is emitted to warn users that the
 | :------------ | :----- | :---------- |
 | `MCP9001` | In place | The `EnumSchema` and `LegacyTitledEnumSchema` APIs are deprecated as of specification version 2025-11-25. Use the current schema APIs instead. |
 | `MCP9002` | Removed | The `AddXxxFilter` extension methods on `IMcpServerBuilder` (e.g., `AddListToolsFilter`, `AddCallToolFilter`, `AddIncomingMessageFilter`) were superseded by `WithRequestFilters()` and `WithMessageFilters()`. |
+| `MCP9003` | In place | The `RequestContext<TParams>(McpServer, JsonRpcRequest)` constructor is obsolete. Use the overload that accepts a `parameters` argument: `RequestContext<TParams>(McpServer, JsonRpcRequest, TParams)`. |

src/Common/Obsoletions.cs

@@ -25,4 +25,8 @@ internal static class Obsoletions
 
     // MCP9002 was used for the AddXxxFilter extension methods on IMcpServerBuilder that were superseded by
     // WithMessageFilters() and WithRequestFilters(). The APIs were removed; do not reuse this diagnostic ID.
+
+    public const string RequestContextParamsConstructor_DiagnosticId = "MCP9003";
+    public const string RequestContextParamsConstructor_Message = "Use the constructor overload that accepts a parameters argument.";
+    public const string RequestContextParamsConstructor_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcp9003";
 }

src/ModelContextProtocol.Core/Server/RequestContext.cs

@@ -18,12 +18,12 @@ public sealed class RequestContext<TParams> : MessageContext
     /// </summary>
     /// <param name="server">The server with which this instance is associated.</param>
     /// <param name="jsonRpcRequest">The JSON-RPC request associated with this context.</param>
-    /// <param name="params">The parameters associated with this request.</param>
+    /// <param name="parameters">The parameters associated with this request.</param>
     /// <exception cref="ArgumentNullException"><paramref name="server"/> or <paramref name="jsonRpcRequest"/> is <see langword="null"/>.</exception>
-    public RequestContext(McpServer server, JsonRpcRequest jsonRpcRequest, TParams @params)
+    public RequestContext(McpServer server, JsonRpcRequest jsonRpcRequest, TParams parameters)
         : base(server, jsonRpcRequest)
     {
-        Params = @params;
+        Params = parameters;
     }
 
     /// <summary>
@@ -32,7 +32,7 @@ public RequestContext(McpServer server, JsonRpcRequest jsonRpcRequest, TParams @
     /// <param name="server">The server with which this instance is associated.</param>
     /// <param name="jsonRpcRequest">The JSON-RPC request associated with this context.</param>
     /// <exception cref="ArgumentNullException"><paramref name="server"/> or <paramref name="jsonRpcRequest"/> is <see langword="null"/>.</exception>
-    [Obsolete("Use the constructor that accepts a params argument.")]
+    [Obsolete(Obsoletions.RequestContextParamsConstructor_Message, DiagnosticId = Obsoletions.RequestContextParamsConstructor_DiagnosticId, UrlFormat = Obsoletions.RequestContextParamsConstructor_Url)]
     public RequestContext(McpServer server, JsonRpcRequest jsonRpcRequest)
         : base(server, jsonRpcRequest)
     {