Mimic string pair fix

[stephenberry]

Fix mimic = std::string not working in std::vector<std::pair<...>> (Issue #2285)

Problem

When a custom type with mimic = std::string was used as a key in std::vector<std::pair<...>>, JSON writing worked correctly but reading failed with an expected_quote error.

struct string_like {
    std::string str;
};

template<>
struct glz::meta<string_like> {
    using mimic = std::string;
    static constexpr auto value = &string_like::str;
};

std::vector<std::pair<string_like, string_like>> v;
// Writing produces: {"key1":"value1","key2":"value2"}
// Reading failed with: expected_quote

Cause

The JSON read code was missing mimics_str_t checks in two locations where pair keys are parsed, while the write code correctly handled this case.

Fix

Added mimics_str_t to the key type checks in include/glaze/json/read.hpp:

1. Line 2776 - from<JSON, pair_t<T>> specialization
2. Line 2382 - from<JSON, readable_array_t<T>> specialization for vector-of-pairs