Tests: mark parser callback in unit-contiguous_parser noexcept

Fixes the -Werror=noexcept failure with GCC 26 (ci_test_standards_gcc (26)).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N5NasEsrzHhSNouxSoEwqz
(cherry picked from commit f279c9aa49)
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-contiguous_parser.cpp

@@ -553,7 +553,7 @@ TEST_CASE("contiguous parser")
         CHECK(json::accept("[1,]", false, true));
         CHECK(json::accept("// c\n[1]", true));
         int calls = 0;
-        const json j = json::parse("[1,2]", [&calls](int /*depth*/, json::parse_event_t /*event*/, json& /*parsed*/)
+        const json j = json::parse("[1,2]", [&calls](int /*depth*/, json::parse_event_t /*event*/, json& /*parsed*/) noexcept
         {
             ++calls;
             return true;