CXX-3198 Miscellaneous improvements to address feedback in #1306

[eramongodb]

Followup to #1306 addressing feedback unrelated to reformatting by ClangFormat.

• The % UINT8_MAX is unnecessary as signed-to-unsigned integral conversion is well-defined. This addresses the unintentional exclusion of the value 0xff. If use of std::rand() is concerning despite its use as non-trivial stub bytes to satisfy the interfaces being demonstrated as examples, we can consider replacing it with equally arbitrary but non-random non-trivial key material instead (e.g. {0xde, 0xad, 0xbe, 0xef, 0x00, ...}).

• The call to bson_strncpy is reduced to its simplest form without redundant calls to strlen or std::min to better match its intended usage. (Note: bson_strncpy guarantees null-termination unlike std::strnlen.)

[ghost]

Looks good, thanks! I appreciate the use of a placeholder key, letting us omit the misleading stub entirely.