Port MCP plugin to mcp 2.x

mcp 2.x removed the WebSocket client and renamed or reshaped several APIs,
which broke the MCP plugin import on fresh installs.

- McpError -> MCPError; tool/result fields are snake_case
  (input_schema, mime_type, is_error).
- Add the Streamable HTTP transport ("streamable_http", also
  "streamable-http"/"http") with headers and timeouts via
  create_mcp_http_client. "websocket" now fails with a clear error
  pointing to streamable_http or sse.
- LoggingClientSession logs the method/params of 2.x request and
  notification objects.
- Detect server capabilities by value; hasattr was always true.
- Handle non-CallToolResult results (e.g. InputRequiredResult).
- Share header env expansion between SSE and Streamable HTTP.
- Require mcp>=2,<3 (replaces the earlier <2 pin); update README and tests.

Verified against real mcp 2.2.0 servers over stdio, SSE and Streamable
HTTP (connect, discover, call tool).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: claude

README.md

@@ -377,7 +377,7 @@ The Model Context Protocol (MCP) plugin enables OptiLLM to connect with MCP serv
 OptiLLM supports both **local** and **remote** MCP servers through multiple transport methods:
 - **stdio**: Local servers (traditional)
 - **SSE**: Remote servers via Server-Sent Events
-- **WebSocket**: Remote servers via WebSocket connections
+- **Streamable HTTP**: Remote servers via the MCP Streamable HTTP transport
 
 #### What is MCP?
 
@@ -449,14 +449,17 @@ The [Model Context Protocol](https://modelcontextprotocol.io/) (MCP) is an open
 }
 ```
 
-**Remote Server (WebSocket) - New Feature:**
+**Remote Server (Streamable HTTP):**
 ```json
 {
   "mcpServers": {
-    "remote-ws": {
-      "transport": "websocket",
-      "url": "wss://api.example.com/mcp",
-      "description": "Remote WebSocket MCP server"
+    "remote-http": {
+      "transport": "streamable_http",
+      "url": "https://api.example.com/mcp",
+      "headers": {
+        "Authorization": "Bearer ${API_TOKEN}"
+      },
+      "description": "Remote Streamable HTTP MCP server"
     }
   },
   "log_level": "INFO"
@@ -482,8 +485,8 @@ The [Model Context Protocol](https://modelcontextprotocol.io/) (MCP) is an open
       "description": "GitHub MCP server"
     },
     "remote-api": {
-      "transport": "websocket",
-      "url": "wss://api.company.com/mcp",
+      "transport": "streamable_http",
+      "url": "https://api.company.com/mcp",
       "description": "Company internal MCP server"
     }
   },
@@ -495,7 +498,7 @@ The [Model Context Protocol](https://modelcontextprotocol.io/) (MCP) is an open
 
 **Common Parameters:**
 - **Server name**: A unique identifier for the server (e.g., "filesystem", "github")
-- **transport**: Transport method - "stdio" (default), "sse", or "websocket"
+- **transport**: Transport method - "stdio" (default), "sse", or "streamable_http"
 - **description** (optional): Description of the server's functionality
 - **timeout** (optional): Connection timeout in seconds (default: 5.0)
 
@@ -509,8 +512,12 @@ The [Model Context Protocol](https://modelcontextprotocol.io/) (MCP) is an open
 - **headers** (optional): HTTP headers for authentication
 - **sse_read_timeout** (optional): SSE read timeout in seconds (default: 300.0)
 
-**websocket Transport (WebSocket):**
-- **url**: The WebSocket endpoint URL
+**streamable_http Transport (Streamable HTTP):**
+- **url**: The MCP endpoint URL
+- **headers** (optional): HTTP headers for authentication
+- **sse_read_timeout** (optional): Read timeout in seconds (default: 300.0)
+
+The `websocket` transport was removed in MCP Python SDK 2.x and is no longer supported; use `streamable_http` or `sse` instead.
 
 **Environment Variable Expansion:**
 Headers and other string values support environment variable expansion using `${VARIABLE_NAME}` syntax. This is especially useful for API keys:
@@ -536,12 +543,12 @@ You can use any of the [official MCP servers](https://modelcontextprotocol.io/ex
 - **SQLite**: `@modelcontextprotocol/server-sqlite` - SQLite database access
 - **Brave Search**: `@modelcontextprotocol/server-brave-search` - Web search capabilities
 
-##### Remote MCP Servers (SSE/WebSocket transport)
+##### Remote MCP Servers (SSE/Streamable HTTP transport)
 
 Remote servers provide centralized access without requiring local installation:
 
 - **GitHub MCP Server**: `https://api.githubcopilot.com/mcp` - Repository management, issue tracking, and code analysis
-- **Third-party servers**: Any MCP server that supports SSE or WebSocket protocols
+- **Third-party servers**: Any MCP server that supports the SSE or Streamable HTTP transports
 
 ##### Example: Comprehensive Configuration
 
@@ -627,7 +634,7 @@ Check this log file for connection issues, tool execution errors, and other diag
 
 2. **Access denied**: For filesystem operations, ensure the paths specified in the configuration are accessible to the process.
 
-**Remote Server Issues (SSE/WebSocket transport):**
+**Remote Server Issues (SSE/Streamable HTTP transport):**
 
 3. **Connection timeout**: Remote servers may take longer to connect. Increase the `timeout` value in your configuration.
 
@@ -641,7 +648,7 @@ Check this log file for connection issues, tool execution errors, and other diag
 
 7. **Method not found**: Some servers don't implement all MCP capabilities (tools, resources, prompts). Verify which capabilities the server supports.
 
-8. **Transport not supported**: Ensure you're using a supported transport: "stdio", "sse", or "websocket".
+8. **Transport not supported**: Ensure you're using a supported transport: "stdio", "sse", or "streamable_http".
 
 **Example: Testing GitHub MCP Connection**