Refactor SAPI generator: decouple argument classes and annotations into namespace sapi.

This change cleans up and modularizes the types and utilities extracted from sandboxed_library_emitter:
- Promotes Arg, PointerArg, CallbackArg, and simple arg types directly into namespace sapi.
- Decouples Annotations and context binding helpers into annotations.{h,cc}.
- Scopes AST manipulation utilities in namespace sapi::ast.
- Streamlines pointer_arg.h by moving method implementations to pointer_arg.cc.
- Relocates kIncludePrefix to arg.h and removes unnecessary ast_utils.h includes.
- Exposes standalone ConvertArg and ConvertArgImpl in arg_converter.h.

There are no functional changes.

PiperOrigin-RevId: 971269415
Change-Id: I37468757a69b6922506e833686b3ac7891a24aed

Author: carl-smith

sandboxed_api/tools/clang_generator/BUILD

@@ -44,6 +44,7 @@ cc_library(
     hdrs = [
         "annotations.h",
         "arg.h",
+        "arg_converter.h",
         "ast_utils.h",
         "callback_arg.h",
         "diagnostics.h",

sandboxed_api/tools/clang_generator/CMakeLists.txt

@@ -24,6 +24,7 @@ add_library(sapi_generator
   annotations.h
   arg.h
   arg_converter.cc
+  arg_converter.h
   ast_utils.cc
   ast_utils.h
   callback_arg.cc