fix: address review feedback — restore timer.Stop() and fix test spacing

- Restore timer.Stop() no-op in case <-timer.C for symmetry with ctx.Done
- Add missing blank line between TestNoInsecureOption_RejectsHTTP and
  TestNoInsecureOption_RejectsUppercaseHTTP
- Remove double blank line before TestAllowInsecureHTTP_WithoutEnvVar_Rejected

Resolves review comments from sonnet-review-bot on PR #113.

github/client.go

@@ -319,6 +319,7 @@ func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept st
 				timer := time.NewTimer(delay)
 				select {
 				case <-timer.C:
+					timer.Stop() // no-op after fire; kept for symmetry with the ctx.Done case
 				case <-ctx.Done():
 					timer.Stop()
 					return nil, ctx.Err()