fix(gitea): address review feedback on diff size limiting

- Add concurrency safety note to MaxDiffSize field documentation,
  mirroring the existing note on RetryBackoff
- Consolidate six individual test functions into a single table-driven
  test (TestGetPullRequestDiff_SizeLimits) reducing repetition
- Add //nolint:errcheck annotation to test handler w.Write calls

gitea/client.go

@@ -71,6 +71,9 @@ type Client struct {
 
 	// MaxDiffSize is the maximum number of bytes allowed when fetching a PR diff.
 	// If zero, defaults to DefaultMaxDiffSize (10 MB). Set to any negative value to disable the limit.
+	//
+	// This field must be configured before the first request is made.
+	// Modifying it while requests are in flight is not safe.
 	MaxDiffSize int64
 }