Move to_bson's binary subtype check into calc_bson_sizes (#5703)

to_bson() rejected a binary value's subtype above 255 (out_of_range.415)
in write_bson_binary(), which only has the binary_t, not the basic_json
value that holds it, so the exception was created with no JSON_DIAGNOSTICS
context even though the equivalent to_msgpack() check names the value's
path. The check also ran after the document size, all preceding elements,
and this element's header and length had already reached the output
adapter, so a caller-provided std::vector or std::string ended up holding
a truncated document.

calc_bson_sizes() already walks every value before anything is written,
to size embedded documents and arrays and to reject invalid keys
(out_of_range.409) up front. The subtype check now runs there instead,
in calc_bson_binary_size(), which is given the basic_json value so the
exception can use it as context. The now-redundant check in
write_bson_binary() is removed, since calc_bson_sizes() always throws
first if any binary value in the document has an oversized subtype.

Fixes #5675.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-diagnostics.cpp

@@ -104,6 +104,12 @@ TEST_CASE("Regression tests for extended diagnostics")
         CHECK_THROWS_WITH_AS(j.unflatten(), "[json.exception.type_error.315] (/~1foo) values in object must be primitive", json::type_error);
     }
 
+    SECTION("Regression test for issue #5675 - to_bson: out_of_range.415 has no diagnostics context")
+    {
+        json const j = {{"a", {{"b", json::binary({1, 2}, 300)}}}};
+        CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.out_of_range.415] (/a/b) subtype 300 is too large for the BSON binary subtype (max 255)", json::out_of_range);
+    }
+
     SECTION("Regression test for issue #2838 - Assertion failure when inserting into arrays with JSON_DIAGNOSTICS set")
     {
         // void push_back(basic_json&& val)