Factor out more repeated error paths and test boilerplate

Library:
- add throw_cannot_use_with() for the 34 copies of type_error.304-312
  "cannot use X with Y"
- iter_impl: add throw_cannot_get_value() (invalid_iterator.214)
- parser: add syntax_error() for the 12 parse_error.101 sites
- ordered_map: share the four at() bodies via at_impl()
- json_sax_dom_callback_parser: add pop_container() for end_object()
  and end_array()
- binary_reader: build the two UBJSON/BJData length-type messages with
  concat() and last_byte_error()

Tests:
- move same_value(), the NDEBUG guard, and step 0 (parse without
  exceptions) of the seven fuzzer drivers into tests/src/fuzzer_common.hpp

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/fuzzer_common.hpp

@@ -0,0 +1,49 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// code shared by the fuzzer drivers tests/src/fuzzer-parse_*.cpp
+
+#pragma once
+
+#include <cassert> // assert
+#include <nlohmann/json.hpp>
+
+// the round-trip checks of the drivers are assertions; NDEBUG would compile them away
+#ifdef NDEBUG
+    #error "the fuzzer drivers must be built without NDEBUG"
+#endif
+
+// compares dumps rather than values, because NaN != NaN; keep writes strings
+// byte for byte, so ill-formed UTF-8 that a binary reader accepts cannot throw
+inline bool same_value(const nlohmann::json& lhs, const nlohmann::json& rhs)
+{
+    return lhs.dump(-1, ' ', false, nlohmann::json::error_handler_t::keep) == rhs.dump(-1, ' ', false, nlohmann::json::error_handler_t::keep);
+}
+
+// step 0 of each driver: parse the input without exceptions; a parse error
+// must then be reported as a discarded value, never thrown. Type and
+// out-of-range errors are not parse errors and still throw; then @a threw is
+// set and null is returned.
+template<typename Parse>
+nlohmann::json parse_without_exceptions(Parse parse, bool& threw)
+{
+    threw = false;
+    try
+    {
+        return parse();
+    }
+    catch (const nlohmann::json::parse_error&)
+    {
+        assert(false);
+    }
+    catch (const nlohmann::json::exception&)
+    {
+        threw = true;
+    }
+    return {};
+}