Refactor genai package to new modular interface

Reorganized the genai package by removing legacy LLM-related files and introducing a new modular interface under the 'interface' directory. Added provider-specific model classes, centralized constants, and updated the example to use the new API and data structures. Updated exports in genai.dart and improved dependency management.

packages/genai/lib/interface/model_providers/openai.dart

@@ -0,0 +1,49 @@
+import 'package:better_networking/better_networking.dart';
+import '../../models/models.dart';
+import '../consts.dart';
+
+class OpenAIModel extends ModelProvider {
+  static final instance = OpenAIModel();
+
+  @override
+  ModelRequestData get defaultRequestData =>
+      kDefaultModelRequestData.copyWith(url: kOpenAIUrl);
+
+  @override
+  HttpRequestModel? createRequest(ModelRequestData? requestData) {
+    if (requestData == null) {
+      return null;
+    }
+    return HttpRequestModel(
+      method: HTTPVerb.post,
+      url: requestData.url,
+      authModel: AuthModel(
+        type: APIAuthType.bearer,
+        bearer: AuthBearerModel(token: requestData.apiKey),
+      ),
+      body: kJsonEncoder.convert({
+        "model": requestData.model,
+        "messages": [
+          {"role": "system", "content": requestData.systemPrompt},
+          if (requestData.userPrompt.isNotEmpty) ...{
+            {"role": "user", "content": requestData.userPrompt},
+          } else ...{
+            {"role": "user", "content": "Generate"},
+          },
+        ],
+        ...requestData.getModelConfigMap(),
+        if (requestData.stream ?? false) ...{'stream': true},
+      }),
+    );
+  }
+
+  @override
+  String? outputFormatter(Map x) {
+    return x["choices"]?[0]["message"]?["content"]?.trim();
+  }
+
+  @override
+  String? streamOutputFormatter(Map x) {
+    return x["choices"]?[0]["delta"]?["content"];
+  }
+}