fix: prevent transient upstream failures from cascading into cooldown 429s

[chenyme]

fix: prevent transient upstream failures from cascading into cooldown 429s

• enable HTTP/2 health checks for all Grok Build transports
• evict idle Build connections before upstream idle timeouts
• apply the transport policy to direct, proxy, SOCKS, and tunnel clients
• treat non-account-scoped upstream 5xx responses as short soft cooldowns
• preserve HTTP status diagnostics without increasing persistent failure counts
• keep genuine 429 handling and Retry-After behavior unchanged
• add coverage for HTTP/2 transport settings, soft cooldowns, and account rotation