Fix silent integer-to-char narrowing on origin_t numeric members

[garethsb]

Assigning a uint64_t to sdp_parameters::origin_t::session_version silently selected utility::string_t::operator=(char), producing a one-character string from the low octet of the right-hand side. This manifests as a corrupted o= line after IS-05 PATCH-driven SDP regeneration, e.g.

o=- 1779263096 \xfc IN IP4 ...

where \xfc is the low byte of the NTP-seconds session-version.

Introduce nmos::numeric_string, a thin wrapper around utility::string_t that exposes two operator= overloads (utility::string_t with valid_numeric_string validation, and uint64_t with to_string_t). Other integer-ish right-hand sides (signed int, char, bool, ...) chain to operator=(uint64_t) and therefore produce the same implicit-conversion diagnostics on integer assignment that a uint64_t member would produce.

Default construction delegates to numeric_string(uint64_t{}) so that origin_t() yields "0" / "0" for session_id / session_version, matching the historic uint64_t behaviour and producing a well-formed o= line from a freshly-constructed origin_t.

Retype origin_t::session_id and origin_t::session_version to numeric_string. The existing string-pair and uint64_t-pair constructors compile unchanged because the wrapper accepts both forms; the explicit to_string_t / valid_numeric_string calls in those constructor bodies are now unnecessary and removed.

Two value_of call sites in make_session_description access .value explicitly because std::pair<string_t, web::json::details::value_init> cannot chain two user-defined conversions
(numeric_string -> string_t -> value_init).

Tighten valid_numeric_string to reject the empty string (previously accepted because std::all_of on an empty range is vacuously true), aligning the big-digits-converter validation with digits2jn which already rejects empty input via istream >> uint64_t.

Upgrade std::isdigit / std::isalnum calls on utility::char_t in valid_numeric_string and cpprest::is_tchar to the locale-aware templated overload std::isdigit(c, std::locale::classic()), removing the latent UB when utility::char_t is wchar_t and c is outside the unsigned-char range; the classic locale gives the same C-style ASCII semantics across narrow and wide char.

Add testNumericString covering default == "0", uint64_t across the full range, leading-zero preservation, empty + non-numeric rejection, and the historic NTP-seconds footgun via origin_t::session_version.