[Python] Fixed the issue with nested unions relying on InitFromBuf. (#7576)

* feat: Fixed the issue with nested unions relying on InitFromBuf.
Problem: Issue #7569
Nested Unions were broken with the introduction of parsing buffers with an initial encoding offset.

Fix:
Revert the InitFromBuf method to the previous version and introduction of InitFromPackedBuf that allows
users to read types from packed buffers applying the offset automatically.

Test:
Added in TestNestedUnionTables to test the encoding and decoding ability using a nested table with a
union field.

* fix: Uncommented generate code command

scripts/generate_code.py

@@ -147,7 +147,7 @@ TS_OPTS = ["--ts", "--gen-name-strings"]
 LOBSTER_OPTS = ["--lobster"]
 SWIFT_OPTS = ["--swift", "--gen-json-emit", "--bfbs-filenames", str(tests_path)]
 SWIFT_OPTS_CODE_GEN = [
-    "--swift", 
+    "--swift",
     "--gen-json-emit",
     "--bfbs-filenames",
     swift_code_gen
@@ -305,14 +305,14 @@ flatc(
 
 # Generate the annotated binary of the monster_test binary schema.
 flatc_annotate(
-    schema="../reflection/reflection.fbs", 
-    file="monster_test.bfbs", 
+    schema="../reflection/reflection.fbs",
+    file="monster_test.bfbs",
     include="include_test"
 )
 
 flatc_annotate(
-    schema="monster_test.fbs", 
-    file="monsterdata_test.mon", 
+    schema="monster_test.fbs",
+    file="monsterdata_test.mon",
     include="include_test"
 )
 
@@ -371,6 +371,12 @@ flatc(
 )
 
 
+flatc(
+    BASE_OPTS + PYTHON_OPTS,
+    schema="nested_union_test.fbs",
+)
+
+
 # Optional Scalars
 optional_scalars_schema = "optional_scalars.fbs"
 flatc(["--java", "--kotlin", "--lobster"], schema=optional_scalars_schema)