fix(mail): drain UidFetch channel on early error; clarify ParseHeaderOnly doc

If header/body parsing errored mid-fetch we returned without draining the
message channel, so the UidFetch goroutine could block on a full channel.
Both fetch paths now break, drain remaining messages, then read the done
error. Verified with the race detector.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/mail/imap.go

@@ -103,7 +103,10 @@ func (c *Client) fetchHeadersByUIDSet(folder string, set *imap.SeqSet) ([]Header
 	done := make(chan error, 1)
 	go func() { done <- c.c.UidFetch(set, items, msgCh) }()
 
-	var out []Header
+	var (
+		out  []Header
+		ferr error
+	)
 	for m := range msgCh {
 		r := m.GetBody(section)
 		if r == nil {
@@ -111,17 +114,26 @@ func (c *Client) fetchHeadersByUIDSet(folder string, set *imap.SeqSet) ([]Header
 		}
 		raw, err := io.ReadAll(r)
 		if err != nil {
-			return nil, fmt.Errorf("uid %d: read header: %w", m.Uid, err)
+			ferr = fmt.Errorf("uid %d: read header: %w", m.Uid, err)
+			break
 		}
 		h, err := ParseHeaderBytes(m.Uid, raw)
 		if err != nil {
-			return nil, err
+			ferr = err
+			break
 		}
 		h.HasAttachments = hasAttachment(m.BodyStructure)
 		out = append(out, h)
 	}
-	if err := <-done; err != nil {
-		return nil, err
+	// Drain any remaining messages so the UidFetch goroutine can finish and
+	// never blocks on a full channel after an early break.
+	for range msgCh {
+	}
+	if err := <-done; err != nil && ferr == nil {
+		ferr = err
+	}
+	if ferr != nil {
+		return nil, ferr
 	}
 	sort.Slice(out, func(i, j int) bool { return out[i].UID > out[j].UID })
 	return out, nil
@@ -144,6 +156,7 @@ func (c *Client) fetchFullByUID(folder string, uid uint32) (Message, error) {
 	var (
 		msg   Message
 		found bool
+		ferr  error
 	)
 	for m := range msgCh {
 		r := m.GetBody(section)
@@ -152,16 +165,24 @@ func (c *Client) fetchFullByUID(folder string, uid uint32) (Message, error) {
 		}
 		raw, err := io.ReadAll(r)
 		if err != nil {
-			return Message{}, fmt.Errorf("uid %d: read body: %w", m.Uid, err)
+			ferr = fmt.Errorf("uid %d: read body: %w", m.Uid, err)
+			break
 		}
 		parsed, err := ParseMessage(m.Uid, raw)
 		if err != nil {
-			return Message{}, err
+			ferr = err
+			break
 		}
 		msg, found = parsed, true
 	}
-	if err := <-done; err != nil {
-		return Message{}, err
+	// Drain so the UidFetch goroutine can finish even after an early break.
+	for range msgCh {
+	}
+	if err := <-done; err != nil && ferr == nil {
+		ferr = err
+	}
+	if ferr != nil {
+		return Message{}, ferr
 	}
 	if !found {
 		return Message{}, fmt.Errorf("uid %d not found in %s", uid, folder)