fix: guard optional accesses and numeric parse calls against absent/malformed values

[SoundMatt]

Problem

Four sites in triple_assembler.cpp and helper.cpp call .value() on optionals without checking .has_value(), or pass WebSocket-derived strings to std::stod/std::stoi without catching conversion errors. Any absent value or non-numeric string from the WebSocket causes std::bad_optional_access or an unhandled std::invalid_argument/std::out_of_range exception.

Root cause

The code assumes that node values and coordinate strings are always present and always numeric. WebSocket messages are external, untrusted input and can legitimately omit values or contain non-numeric data.

Fix

(a) Guard node.getValue().value() in generateTriplesFromNode with a has_value() check and throw std::runtime_error if absent. (b) Guard the latitude/longitude getValue().value() calls in generateTriplesFromCoordinates the same way. (c) Wrap std::stod in getCoordInNtm in try/catch, returning std::nullopt on failure. (d) Wrap std::stoi in parseISO8601ToTime in try/catch, assigning std::nullopt to milliseconds on failure.