fix: ignore non-positive delay in SetExpiresAfter

A zero or negative delay would silently produce an expires value in the
past or be treated as "unset" inconsistently. Guard the assignment so
only positive delays take effect, matching the semantics of the existing
expires == 0 sentinel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yaronf

config.go

@@ -66,7 +66,9 @@ func (c *SignConfig) SetExpires(expires int64) *SignConfig {
 // Use this for a relative validity window instead of an absolute timestamp.
 // Default: 0 (do not add the parameter).
 func (c *SignConfig) SetExpiresAfter(delay int64) *SignConfig {
-	c.expiresAfter = delay
+	if delay > 0 {
+		c.expiresAfter = delay
+	}
 	return c
 }