fix: disable keep alive to avoid stale socket reuse

[ulemons]

Changes proposed ✍️

What:

• Introduced a static https.Agent with keepAlive: false.
• This disables HTTP keep-alive for all outgoing requests in our TinybirdClient.

Why:

• We observed recurring socket hang up errors caused by stale socket reuse.
• By disabling keep-alive, each request uses a fresh TCP/TLS connection, preventing the client from reusing sockets that may already have been closed by the server or a load balancer.
• This should significantly reduce the frequency of connection reset errors.

Checklist ✅

• Label appropriately with Feature, Improvement, or Bug.