feat: add ctx.prompt.form() API for multi-field form dialogs

Add PromptFormRequest and PromptFormResponse types to enable plugins to
display forms with multiple input fields. Implement the form() method in
the prompt context and wire up frontend event handling to show and collect
form responses from users.

packages/plugin-runtime/src/PluginInstance.ts

@@ -28,6 +28,7 @@ import type {
   ListHttpRequestsResponse,
   ListWorkspacesResponse,
   PluginContext,
+  PromptFormResponse,
   PromptTextResponse,
   RenderGrpcRequestResponse,
   RenderHttpRequestResponse,
@@ -661,6 +662,13 @@ export class PluginInstance {
           });
           return reply.value;
         },
+        form: async (args) => {
+          const reply: PromptFormResponse = await this.#sendForReply(context, {
+            type: 'prompt_form_request',
+            ...args,
+          });
+          return reply.values;
+        },
       },
       httpResponse: {
         find: async (args) => {