fix: forward original tool name to remote MCP servers

[null-runner]

Problem

When tool-name-prefix is enabled (or when using explicit prefix in server config), the gateway correctly prefixes tool names for the client (e.g., new_page → server__new_page).

However, when forwarding the tools/call request to the remote MCP server, the prefixed name was being sent instead of the original name. The remote server doesn't know about gateway prefixes and fails with "tool not found".

Root Cause

In handlers.go, the mcpServerToolHandler was using req.Params.Name (the prefixed name from the client request) when calling client.Session().CallTool():

params := &mcp.CallToolParams{
    Name: req.Params.Name,  // Bug: sends "server__new_page" to server that only knows "new_page"
    ...
}

Solution

Pass the original tool name (without prefix) to the handler and use it when forwarding to the remote server:

1. Add originalToolName parameter to mcpServerToolHandler
2. Pass tool.Name (original name) when creating the handler in listCapabilities
3. Use originalToolName instead of req.Params.Name when calling the remote server

Testing

Tested with Chrome DevTools MCP server using custom prefix configuration:

• Before fix: chromedev__new_page → "Tool new_page not found"
• After fix: chromedev__new_page → works correctly

Related

This fix is complementary to #263 which changes the prefix separator from : to __. Without this fix, the tool-name-prefix feature doesn't work regardless of which separator is used.

[null-runner]

Closing for further testing before resubmitting.