Phase 5 follow-up: discover the \Sent mailbox; record the approved real send (AUD-71)

The approved real test reply was sent and the thread updated (same
X-GM-THRID as the original). The live test exposed one bug: the thread
check hardcoded Gmail's sent mailbox as "[Gmail]/Sent Mail", but this
account exposes it as "[Google Mail]/Sent Mail".

- internal/mailfetch: LIST parser, ListMailboxes, and SentMailbox that
  finds the mailbox flagged \Sent (RFC 6154) with name fallbacks.
- internal/mailops: VerifyThreadUpdated selects the discovered \Sent
  mailbox instead of a hardcoded name.
- cmd/email-agent: -check-thread <id> exercises the check against real
  Gmail without sending mail.
- Tests for LIST parsing, sent-mailbox discovery, and the discovery
  error path.
- docs/phase5-replies.md: record send-as verification, the sent reply's
  thread id, the app confirmation (sent_reply_in_thread=true), and the
  fix.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/mailfetch/mailbox.go

@@ -0,0 +1,118 @@
+// Mailbox discovery for the IMAP client.
+//
+// Gmail exposes its special-use mailboxes with RFC 6154 attributes. The Sent
+// mailbox is named "[Gmail]/Sent Mail" on most accounts but "[Google Mail]/Sent
+// Mail" on others (and "Sent" on non-Gmail servers). Phase 5 asks the server for
+// the mailbox flagged \Sent instead of hardcoding a name.
+package mailfetch
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Mailbox is one entry from an IMAP LIST response.
+type Mailbox struct {
+	Name       string
+	Attributes []string
+}
+
+// Has reports whether the mailbox carries the given attribute (for example
+// `\Sent` or `\All`). The comparison is case-insensitive.
+func (m Mailbox) Has(attr string) bool {
+	for _, a := range m.Attributes {
+		if strings.EqualFold(a, attr) {
+			return true
+		}
+	}
+	return false
+}
+
+// ListMailboxes returns every mailbox the server advertises.
+func (c *Client) ListMailboxes() ([]Mailbox, error) {
+	var out []Mailbox
+	err := c.exec(`LIST "" "*"`, func(r *response) error {
+		if mb, ok := parseList(r.line); ok {
+			out = append(out, mb)
+		}
+		return nil
+	})
+	return out, err
+}
+
+// sentFallbacks are used when the server does not advertise the \Sent
+// attribute.
+var sentFallbacks = []string{
+	"[Gmail]/Sent Mail",
+	"[Google Mail]/Sent Mail",
+	"Sent",
+	"Sent Mail",
+}
+
+// SentMailbox returns the mailbox the server flags \Sent. It falls back to the
+// common names when the attribute is absent.
+func (c *Client) SentMailbox() (string, error) {
+	boxes, err := c.ListMailboxes()
+	if err != nil {
+		return "", err
+	}
+	for _, mb := range boxes {
+		if mb.Has(`\Sent`) {
+			return mb.Name, nil
+		}
+	}
+	for _, want := range sentFallbacks {
+		for _, mb := range boxes {
+			if strings.EqualFold(mb.Name, want) {
+				return mb.Name, nil
+			}
+		}
+	}
+	return "", fmt.Errorf("no sent mailbox advertised by the server")
+}
+
+// parseList parses one "* LIST (attrs) delim name" response line. XLIST is
+// accepted as well, for servers that only implement the older extension.
+func parseList(line string) (Mailbox, bool) {
+	upper := strings.ToUpper(line)
+	if !strings.HasPrefix(upper, "* LIST ") && !strings.HasPrefix(upper, "* XLIST ") {
+		return Mailbox{}, false
+	}
+	open := strings.IndexByte(line, '(')
+	if open < 0 {
+		return Mailbox{}, false
+	}
+	closeIdx := strings.IndexByte(line[open:], ')')
+	if closeIdx < 0 {
+		return Mailbox{}, false
+	}
+	closeIdx += open
+	attrs := strings.Fields(line[open+1 : closeIdx])
+	tail := strings.TrimSpace(line[closeIdx+1:])
+
+	name := lastQuoted(tail)
+	if name == "" {
+		fields := strings.Fields(tail)
+		if len(fields) == 0 {
+			return Mailbox{}, false
+		}
+		name = strings.Trim(fields[len(fields)-1], `"`)
+	}
+	if name == "" {
+		return Mailbox{}, false
+	}
+	return Mailbox{Name: name, Attributes: attrs}, true
+}
+
+// lastQuoted returns the contents of the last "..." segment in s.
+func lastQuoted(s string) string {
+	end := strings.LastIndexByte(s, '"')
+	if end <= 0 {
+		return ""
+	}
+	start := strings.LastIndexByte(s[:end], '"')
+	if start < 0 {
+		return ""
+	}
+	return s[start+1 : end]
+}

internal/mailfetch/mailbox_test.go

@@ -0,0 +1,77 @@
+package mailfetch
+
+import "testing"
+
+func TestParseList(t *testing.T) {
+	// Gmail on this account advertises "[Google Mail]/..." rather than
+	// "[Gmail]/...". The \Sent attribute is what matters.
+	cases := []struct {
+		line string
+		name string
+		attr string
+		ok   bool
+	}{
+		{`* LIST (\HasChildren \Noselect) "/" "[Gmail]"`, "[Gmail]", `\Noselect`, true},
+		{`* LIST (\HasNoChildren \Sent) "/" "[Google Mail]/Sent Mail"`, "[Google Mail]/Sent Mail", `\Sent`, true},
+		{`* LIST (\HasNoChildren) "/" "Replied-to/hello@where-woof.com"`, "Replied-to/hello@where-woof.com", `\HasNoChildren`, true},
+		{`* LIST (\All \HasNoChildren) "/" "[Google Mail]/All Mail"`, "[Google Mail]/All Mail", `\All`, true},
+		{`* LIST (\HasNoChildren) NIL "Sent"`, "Sent", `\HasNoChildren`, true},
+		{`* OK no list here`, "", "", false},
+	}
+	for _, tc := range cases {
+		mb, ok := parseList(tc.line)
+		if ok != tc.ok {
+			t.Errorf("parseList(%q) ok = %v, want %v", tc.line, ok, tc.ok)
+			continue
+		}
+		if !ok {
+			continue
+		}
+		if mb.Name != tc.name {
+			t.Errorf("parseList(%q) name = %q, want %q", tc.line, mb.Name, tc.name)
+		}
+		if !mb.Has(tc.attr) {
+			t.Errorf("parseList(%q) missing attribute %q (got %v)", tc.line, tc.attr, mb.Attributes)
+		}
+	}
+}
+
+func TestListMailboxesAndSentMailbox(t *testing.T) {
+	script := "* LIST (\\HasChildren \\Noselect) \"/\" \"[Gmail]\"\r\n" +
+		"* LIST (\\HasNoChildren) \"/\" \"Replied-to/hello@where-woof.com\"\r\n" +
+		"* LIST (\\All \\HasNoChildren) \"/\" \"[Google Mail]/All Mail\"\r\n" +
+		"* LIST (\\HasChildren \\Sent) \"/\" \"[Google Mail]/Sent Mail\"\r\n" +
+		"A0001 OK LIST completed\r\n"
+	c, fc := newFakeClient(script)
+
+	boxes, err := c.ListMailboxes()
+	if err != nil {
+		t.Fatalf("ListMailboxes() error = %v", err)
+	}
+	if len(boxes) != 4 {
+		t.Fatalf("ListMailboxes() = %d, want 4", len(boxes))
+	}
+	if got := fc.w.String(); got == "" || !contains(got, `LIST "" "*"`) {
+		t.Errorf("command sent = %q, want LIST", got)
+	}
+
+	// SentMailbox issues its own LIST, so it needs a fresh script (a live
+	// connection would simply answer again).
+	c2, _ := newFakeClient(script)
+	sent, err := c2.SentMailbox()
+	if err != nil {
+		t.Fatalf("SentMailbox() error = %v", err)
+	}
+	if sent != "[Google Mail]/Sent Mail" {
+		t.Errorf("SentMailbox() = %q, want [Google Mail]/Sent Mail", sent)
+	}
+}
+
+func contains(haystack, needle string) bool {
+	for i := 0; i+len(needle) <= len(haystack); i++ {
+		if haystack[i:i+len(needle)] == needle {
+			return true
+		}
+	}
+	return false
+}