fix(github): address review findings from round 2880/2883

Sonnet MINOR #1: Stop timer after <-timer.C fires for idiomatic cleanup.
Sonnet MINOR #2: Document that empty array from contents API is valid (empty dir).
Sonnet MINOR #3: Document that GetPullRequestFiles returns nil for no files.
Sonnet NIT #4: Strengthen SetHTTPClient/SetRetryBackoff docs to clarify test-only intent.
Sonnet NIT #5: Document GetCommitStatuses fail-fast behavior.
Sonnet NIT #6: Document double-slash collapsing in escapePath.
Security MINOR #1: Document redirect policy responsibility when providing custom client.
Security MINOR #2: Reduce maxErrorBodyBytes from 64KB to 4KB to limit sensitive data exposure.

github/files.go

@@ -42,6 +42,8 @@ func (c *Client) ListContents(ctx context.Context, owner, repo, path string) ([]
 
 	// The GitHub contents API returns an array for directories and an object
 	// for single files. Try array first (common case), then fall back to object.
+	// An empty array ([]) is valid — it represents an empty directory — and
+	// results in a zero-length slice returned without error.
 	var entries []entry
 	if err := json.Unmarshal(body, &entries); err != nil {
 		var single entry
@@ -69,11 +71,12 @@ func (c *Client) ListContents(ctx context.Context, owner, repo, path string) ([]
 
 // escapePath escapes each segment of a relative file path for use in URLs.
 // Slashes are preserved as path separators; other special characters are escaped.
-// Dot-segments ("." and "..") are silently removed to prevent path traversal.
-// This is intentional: callers may receive a different path than requested without
-// error. The function is package-private, and all callers (GetFileContentAtRef,
-// ListContents) already handle missing-file errors from the API if the cleaned
-// path doesn't match what the caller intended.
+// Dot-segments ("." and "..") and empty segments (from consecutive slashes like
+// "a//b") are silently removed to prevent path traversal and produce canonical
+// paths. This is intentional: callers may receive a different path than requested
+// without error. The function is package-private, and all callers
+// (GetFileContentAtRef, ListContents) already handle missing-file errors from the
+// API if the cleaned path doesn't match what the caller intended.
 func escapePath(p string) string {
 	parts := strings.Split(p, "/")
 	var clean []string