CDRIVER-5759 fix relaxed encoding of date after year 9999 (#1772)

Use format `{"$date": {"$numberLong": "<ms from epoch>"}}` rather than `{"$date": "<ISO-8601 date+time>" }`

Author: kevinAlbs

src/libbson/src/bson/bson.c

@@ -2619,7 +2619,10 @@ _bson_as_json_visit_date_time (const bson_iter_t *iter, const char *key, int64_t
    BSON_UNUSED (iter);
    BSON_UNUSED (key);
 
-   if (state->mode == BSON_JSON_MODE_CANONICAL || (state->mode == BSON_JSON_MODE_RELAXED && msec_since_epoch < 0)) {
+   const int64_t msec_since_Y10K = 253402300800000; // Milliseconds since 10000-01-01T00:00:00Z.
+
+   if (state->mode == BSON_JSON_MODE_CANONICAL ||
+       (state->mode == BSON_JSON_MODE_RELAXED && (msec_since_epoch < 0 || msec_since_epoch >= msec_since_Y10K))) {
       mcommon_string_append (state->str, "{ \"$date\" : { \"$numberLong\" : \"");
       mcommon_string_append_printf (state->str, "%" PRId64, msec_since_epoch);
       mcommon_string_append (state->str, "\" } }");

src/libbson/tests/json/bson_corpus/datetime.json

@@ -24,6 +24,7 @@
         {
             "description" : "Y10K",
             "canonical_bson" : "1000000009610000DC1FD277E6000000",
+            "relaxed_extjson" : "{\"a\":{\"$date\":{\"$numberLong\":\"253402300800000\"}}}",
             "canonical_extjson" : "{\"a\":{\"$date\":{\"$numberLong\":\"253402300800000\"}}}"
         },
         {