address review feedback: baseURL TODO, timer-cancel test, fast retry test, doc note

- Add TODO comment on unused baseURL field explaining its intended
  future use by higher-level exported methods
- Rewrite TestDoRequest_ContextCanceled to actually exercise the
  timer-cancel path in the retry select (goroutine cancels context
  while client is blocked in timer.C select)
- Change Retry-After: 1 to Retry-After: 0 in IntegerSeconds test
  to avoid unnecessary 1s sleep during test runs
- Add doc note on SetRetryBackoff explaining that an empty non-nil
  slice silently drops Retry-After delays

github/client.go

@@ -89,6 +89,10 @@ func asAPIError(err error) (*APIError, bool) {
 // SetHTTPClient and SetRetryBackoff are intended for test setup only and must
 // be called before any goroutines issue requests; they have no synchronization.
 type Client struct {
+	// TODO: baseURL is populated by NewClient but not yet consumed by doRequest/doGet.
+	// Higher-level exported methods (GetPullRequest, etc.) will use it to
+	// construct request URLs; remove this field if those methods end up
+	// accepting full URLs instead.
 	baseURL string
 	token   string
 	http    *http.Client
@@ -126,6 +130,11 @@ func (c *Client) SetHTTPClient(hc *http.Client) {
 
 // SetRetryBackoff sets the delays between retry attempts.
 // This is intended for testing to speed up retry tests.
+//
+// Note: if an empty non-nil slice is provided, Retry-After delays parsed from
+// server responses will be computed and capped but not applied (because
+// attempt < len(backoff) is always false). This is acceptable for the
+// test-only use case but callers should be aware of this edge case.
 func (c *Client) SetRetryBackoff(backoff []time.Duration) {
 	c.retryBackoff = backoff
 }