feat: deferred allocator pattern and configurable formatting support

- **BREAKING**: Remove allocator parameter from public APIs in gql_code_builder
- Use Code.scope pattern for deferred type allocation instead of passing allocator through all functions
- Add configurable formatting support with dart_style 3.0.0
- Add format configuration options (page_width, language_version) in build.yaml
- Pass DartFormatter through all builders for consistent formatting
- Update to gql_code_builder 0.15.0 and gql_build 0.13.0
- Add compatibility with build 3.0.0

This refactoring improves separation of concerns between code generation logic
and import management, making the code cleaner and more testable.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: knaeckeKami

.claude/settings.local.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://json.schemastore.org/claude-code-settings.json",
+  "enableAllProjectMcpServers": false,
+  "permissions": {
+    "allow": [
+      "Bash(gh pr:*)"
+    ]
+  }
+}

codegen/end_to_end_test/build.yaml

@@ -3,6 +3,10 @@ targets:
     builders:
       gql_build|ast_builder:
         enabled: true
+        options:
+          format:
+            page_width: 80
+            language_version: '3.7.0'
       gql_build|req_builder:
         enabled: true
         options: