Fix staj_event::as_double silently returning 0 for non-numeric strings

[jsutes]

Summary

basic_staj_event::as_double silently returns 0.0 (with no error) when a
string_value/key event holds text that isn't a number. Any decoder that
asks a string event for a double — e.g.
decode_csv<std::vector<std::vector<double>>> over infer_types(true) output —
therefore coerces non-numeric and empty cells to 0.0 instead of reporting a
conversion error.

Root cause

Regression introduced in caacd25 ("Remove chars_to, replace with to_double").

Before that commit, the string branch of as_double used the chars_to
functor, which threw json_runtime_error<std::invalid_argument> on invalid
input. The commit migrated to the new out-parameter API:

-                jsoncons::utility::chars_to f;
-                return f(value_.string_data_, length_);
+                double val{0};
+                jsoncons::utility::to_double(value_.string_data_, length_, val);
+                return val;

The new call reports failure through its return value, but this call site
discards it — so val keeps its default 0 and the caller's std::error_code&
is never set. (Later renames turned to_double into decstr_to_double; the
behavioral bug is unchanged.)

For comparison, basic_csv_parser::end_value_with_numeric_check uses the same
decstr_to_double call and does check result.ec.

Fix

include/jsoncons/staj_event.hpp, as_double:

double val{0};
auto result = jsoncons::decstr_to_double(value_.string_data_, length_, val);
if (!result)
{
    ec = conv_errc::not_double;
}
return val;

to_number_result has an explicit operator bool() that is false on error, so
!result covers both std::errc::invalid_argument (non-numeric / empty input)
and any other failure.

Reproducer

#include <jsoncons_ext/csv/csv.hpp>
#include <vector>
#include <string>

int main() {
    auto opt = jsoncons::csv::csv_options{}
        .mapping_kind(jsoncons::csv::csv_mapping_kind::n_rows)
        .assume_header(false);

    // Expected: an error. Actual (before this fix): silent {{0}}.
    auto r = jsoncons::csv::try_decode_csv<std::vector<std::vector<double>>>(
        std::string("hey\n"), opt);
    return r ? 1 : 0;   // returns 1 (success) before the fix
}

Tests

Added TEST_CASE("decode_csv non-numeric string into double reports error")
to test/csv/src/encode_decode_csv_tests.cpp, covering a non-numeric token
("hey\n") and an empty trailing field ("0,\n"). Each section asserts both
try_decode_csv returns an error and decode_csv throws.

• The new test fails on current master (the try_decode_csv call silently
  succeeds).
• With the fix, the new test passes and the full unit-test suite is green
  (883 test cases, 20,639 assertions).