Add cpp_vec_type attribute for object API vector customization

Mirrors cpp_str_type and cpp_ptr_type: a per-field cpp_vec_type attribute
lets users substitute any std::vector-compatible container in generated
T-structs, and --cpp-vec-type sets the global default.

NativeVector() resolves the attribute (falling back to the global option,
then std::vector). Changes applied to GenTypeNative, GenMember, GenParam
(CreateDirect), and the CreateVectorOfStrings fast-path guard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/generate_code.py

@@ -383,6 +383,11 @@ flatc(
     schema="native_type_test.fbs",
 )
 
+flatc(
+    ["--cpp", "--gen-compare", "--gen-mutable", "--gen-object-api", "--reflect-names"],
+    schema="cpp_vec_type_test.fbs",
+)
+
 flatc(
     [
         "--cpp",