[Client] Reject non-positive timeouts in Configuration

[chr-hertel]

Client\Configuration accepts any integer for initTimeout and requestTimeout, so setInitTimeout(0) or setRequestTimeout(-5) silently builds a client that can never succeed — the pending-request check is time() - $timestamp >= $timeout, which is true on the first poll, turning every request into an immediate timeout.

Both are now required to be positive, matching how StdioTransport and HttpTransport validate their buffer caps. Zero is rejected along with negatives: it reads like "no timeout" but actually means "expire immediately".

Validation lives in Configuration rather than the builder setters because Configuration is public API and Client accepts one directly, so the builder is not the only way in.