Add public reusable Flush API

[marandaneto]

Summary

Add a public reusable Flush() API to the Go SDK.

Spec

• sdk-specs/openspec/specs/flush/spec.md

Current evidence

The SDK audit found Client exposes Close() / CloseWithContext(), but no reusable immediate flush API. Close() drains during shutdown, but it also closes/disables the client.

Expected behavior

• Expose a public Flush() method that immediately sends queued events without closing the client.
• Ensure the client remains usable after a successful flush.
• Respect errors/timeouts according to existing Go SDK conventions.
• Add tests that queued events are delivered and later events can still be enqueued.