json_view: pass accept()'s validate-only request as a parameter

Address review of #5970:
- build() takes validate_only as an argument; the validate_only members of
  document_data and basic_json_document are removed, so neither grows
- the internal read path takes an on_error mode (discard, raise, validate)
  instead of allow_exceptions; accept() calls it with on_error::validate
- build_with() has no default for Store
- drop the unneeded static_casts in push_node()
- name the NulIsEnd argument so astyle keeps build<...> intact
- explain in the docs and code why comments/trailing commas keep the index

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasF8Toq7uBWUMCrSjXV

tests/src/unit-json_view_builder.cpp

@@ -59,7 +59,7 @@ built build(const std::string& text, bool comments, bool trailing_commas, bool s
         r.copy.assign(text.begin(), text.end());
         src = r.copy.data();
     }
-    r.ok = nlohmann::detail::view::build < FloatType, !nlohmann::detail::abi_config::strict_nul_handling > (*r.data, src, text.size(), comments, trailing_commas, sentinel, r.failure);
+    r.ok = nlohmann::detail::view::build < FloatType, !nlohmann::detail::abi_config::strict_nul_handling > (*r.data, src, text.size(), comments, trailing_commas, sentinel, false, r.failure);
     r.data->src = src;
     r.data->base[0] = src;
     r.data->base[1] = r.data->arena.data();