fix: don't cache transient errors in version negotiation

Replace sync.Once with sync.Mutex so that only successful version
lookups are cached. Errors (context cancellation, network blips) are
returned without caching, allowing subsequent calls to retry.

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