fix(liveliness): escape '/' in backend names embedded in key expressions

[YuanYuYuan]

Summary

Escape / in escape_backend_field() so backend names or metadata containing a forward slash do not corrupt the liveliness key expression in #930.

Key Changes

• rmw_zenoh_cpp/src/detail/liveliness_utils.cpp — add case '/': out += "%2F"; break; to escape_backend_field().

Rationale

The design note for the optional <backends> keyexpr component lists :, ;, and / as the reserved separators that backend fields must not contain verbatim. unescape_backend_field() already handles %2F correctly via its generic %XX decoder, so the encode/decode pair becomes symmetric with this change.

Without this escape, a backend plugin whose get_backend_type() returns e.g. "cuda/v2" or "vendor/gpu" would embed a literal / in the liveliness token, which the keyexpr parser treats as a field separator — silently corrupting the <backends> component for every peer.

Breaking Changes

None. / is not currently produced by any known backend, so this only closes a latent correctness hole. Existing tokens without / in backend fields are byte-identical before and after this change.

Did you use Generative AI?

Yes. Claude (claude-sonnet-4-6) via Claude Code was used to assist with creating an initial prototype version of the changes contained in this PR.