Move isset data to tuple tail

Summary:
Store immutable thrift-python isset bytes as a trailing tuple element instead of at index 0, so field values are always zero-based whether or not isset inspection is enabled. This removes the field-start offset from the Python and C++ runtime paths, updates the public inspection helpers to read the trailing bytes, and updates the Python CAPI codegen to emit zero-based tuple positions.

It also collapses the two immutable tuple container policies into a single `TupleContainer`, making the trailing isset array an orthogonal `WithIsset` template parameter on the container-creation helpers rather than a separate type.

Reviewed By: iahs

Differential Revision: D110348797

fbshipit-source-id: 4293c5998c54361599a29da992c5e421748fc7a5

Author: ahilger

thrift/compiler/generate/t_mstch_python_capi_generator.cc

@@ -576,11 +576,9 @@ class t_mstch_python_capi_generator : public t_whisker_generator {
       return true;
     });
 
-    def.property("tuple_positions", [this](const t_structured& self) {
+    def.property("tuple_positions", [](const t_structured& self) {
       std::unordered_map<const t_field*, int64_t> field_tuple_indexes;
-      // thrift-python reserves tuple element 0 for the isset byte array only
-      // for non-exceptions that support `get_locally_set_fields`.
-      int64_t tuple_index = enable_get_locally_set_fields(self) ? 1 : 0;
+      int64_t tuple_index = 0;
       for (const t_field* f : self.fields_id_order()) {
         field_tuple_indexes[f] = tuple_index++;
       }

thrift/compiler/test/fixtures/py3/out/python_capi/gen-python-capi/module/thrift_types_capi.cpp

@@ -35,7 +35,7 @@ bool ensure_module_imported() {
     0, 1, 2, 3
   };
   static constexpr std::int16_t _fbthrift__IssetInspectionStruct__tuple_pos[3] = {
-    1, 2, 3
+    0, 1, 2
   };
   static constexpr std::int16_t _fbthrift__HiddenException__tuple_pos[1] = {
     0

thrift/compiler/test/fixtures/python_capi/out/python_capi_isset_deprecated_serialized_dep/gen-python-capi/serialized_dep/thrift_types_capi.cpp

@@ -23,7 +23,7 @@ bool ensure_module_imported() {
   return import();
 }
   static constexpr std::int16_t _fbthrift__MarshalStruct__tuple_pos[4] = {
-    1, 2, 3, 4
+    0, 1, 2, 3
   };
   static constexpr std::int16_t _fbthrift__MarshalError__tuple_pos[3] = {
     0, 1, 2