Use accessor method in server_context_with_meta instead of ivar

## Motivation and Context

`server_context_with_meta` reads `@server_context` (the instance variable)
directly instead of calling the `server_context` accessor method.
This prevents subclasses from overriding `server_context`. Ffor example,
to provide thread-local context in multi-threaded servers like Puma.

Fixes #271.

## How Has This Been Tested?

Added a regression test and passed.

## Breaking Changes

None. This restores the expected behavior of `attr_accessor :server_context`.

lib/mcp/server.rb

@@ -531,14 +531,14 @@ module MCP
 
     def server_context_with_meta(request)
       meta = request[:_meta]
-      if meta && @server_context.is_a?(Hash)
-        context = @server_context.dup
+      if meta && server_context.is_a?(Hash)
+        context = server_context.dup
         context[:_meta] = meta
         context
-      elsif meta && @server_context.nil?
+      elsif meta && server_context.nil?
         { _meta: meta }
       else
-        @server_context
+        server_context
       end
     end
   end