refactor: merge RuntimeVersion and CurrentAPIVersion into RuntimeAPIVersion

After API negotiation, Compose should only rely on the negotiated version
and never use the daemon's raw max version for request shaping. Merge both
functions into a single RuntimeAPIVersion that negotiates via Ping and
returns ClientVersion, erroring if the client reports an empty version
instead of silently falling back to ServerVersion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>