feat: Add context argument to completion handlers

This commit adds a `context` argument to the completion handlers in `PromptsTab`, `ResourcesTab`, `useCompletionState`, and `useConnection`.

The context argument allows passing additional information to the completion engine, which can be used to improve the quality of the completions.  The context is passed as a dictionary of key-value pairs.

The `ResourcesTab` component was also updated to pass `templateValues` as the `context` to the completion handler.

The `useConnection` hook was updated to include the context in the completion request sent to the server.

client/src/components/PromptsTab.tsx

@@ -48,6 +48,7 @@ const PromptsTab = ({
     ref: PromptReference | ResourceReference,
     argName: string,
     value: string,
+    context?: Record<string, string>,
   ) => Promise<string[]>;
   completionsSupported: boolean;
   promptContent: string;