server_context_with_meta reads @server_context ivar instead of accessor method

[AndyGauge]

Problem

server_context_with_meta in lib/mcp/server.rb reads @server_context (the instance variable) directly instead of calling the server_context accessor method:

def server_context_with_meta(request)
  meta = request[:_meta]
  if meta && @server_context.is_a?(Hash)
    context = @server_context.dup
    context[:_meta] = meta
    context
  elsif meta && @server_context.nil?
    { _meta: meta }
  else
    @server_context
  end
end

This means subclasses that override server_context have their override bypassed. The attr_accessor :server_context on line 50 suggests the intent is to use the method, but the implementation reads the ivar.

Use case

We run an MCP server under Puma (multi-threaded). We need server_context to be thread-local so concurrent requests don't clobber each other's user context:

class HireMcpServer < MCP::Server
  def server_context
    Thread.current[:mcp_server_context]
  end

  def server_context=(ctx)
    Thread.current[:mcp_server_context] = ctx
    @server_context = ctx  # workaround: also write ivar for server_context_with_meta
  end
end

Without the @server_context = ctx workaround, tools receive nil for server_context because server_context_with_meta reads the ivar (which is never set) instead of calling our overridden method.

Suggested fix

Replace @server_context with server_context in server_context_with_meta:

def server_context_with_meta(request)
  meta = request[:_meta]
  if meta && server_context.is_a?(Hash)
    context = server_context.dup
    context[:_meta] = meta
    context
  elsif meta && server_context.nil?
    { _meta: meta }
  else
    server_context
  end
end

This is standard Ruby convention — when an attr_accessor is defined, internal code should use the method so subclasses can override behavior.