feat(mailsync): FetchMailFolder — integration-layer walk for ETL consumers

Adds the high-level mail folder walk that sync pipelines need on top of
the raw mail API (list -> get -> download-attachment), so consumers stop
re-implementing it against private client copies.

  type MailSyncMessage struct { ID, Folder, Subject, From, Date, IsNew,
                                HasAttachments, Attachments }
  type MailSyncAttachment struct { ID, Name, Size, Body }
  func (c *Client) FetchMailFolder(ctx, folderID, MailSyncOptions)
                                   ([]MailSyncMessage, error)

Options: Limit / StartIndex for checkpointed walks, FetchBodies to
eagerly download attachment bytes via download.ashx (session-cookie path).

Hydration details:
- list items may omit the attachment array; when hasAttachments is set
  the full record is fetched and its attachments merged
- attachment ids accepted from id/fileId/attachmentId variants
- timestamps parsed from RFC3339 (any fractional digits) and
  second-precision forms

This is the first step of the 2dph integration layer (#1): the brain's
mail-ingest pipeline can now drop its private OOClient copy and consume
this canonical walk directly.

Tests: httptest-backed coverage for pagination, hydration with
full-record fallback, body download incl. auth-cookie requirement,
Limit/StartIndex windows, timestamp parsing.

mailsync.go

@@ -0,0 +1,175 @@
+package onlyoffice
+
+// High-level mail folder walk for ETL consumers (2dph brain mail-ingest,
+// cv tools). This is the "integration layer" half of reusing the canonical
+// client instead of private per-project OOClient copies: the caller gets a
+// single hydrated stream instead of hand-rolling list → get → download
+// against the raw API.
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"time"
+)
+
+// MailSyncAttachment is one attachment of a hydrated mail message.
+type MailSyncAttachment struct {
+	ID   string // id accepted by Client.DownloadMailAttachment
+	Name string
+	Size int64
+	Body []byte // non-nil only when MailSyncOptions.FetchBodies is set
+}
+
+// MailSyncMessage is a hydrated mail message for sync pipelines.
+type MailSyncMessage struct {
+	ID             int64
+	Folder         int
+	Subject        string
+	From           string // raw RFC 5322 header value ("Name" <addr>)
+	Date           time.Time
+	IsNew          bool
+	HasAttachments bool
+	Attachments    []MailSyncAttachment
+}
+
+// MailSyncOptions controls FetchMailFolder.
+type MailSyncOptions struct {
+	Limit       int  // max messages to hydrate; 0 = whole folder
+	StartIndex  int  // skip this many messages before collecting
+	FetchBodies bool // eagerly download attachment bytes
+}
+
+// FetchMailFolder walks a mail folder page by page and hydrates every
+// message: list → get → (optionally) download attachments. It is the single
+// entry point sync pipelines need on top of the mail API.
+//
+// Messages are returned in API order (newest first). The folder walk stops
+// at the first empty or short page.
+func (c *Client) FetchMailFolder(ctx context.Context, folderID int, opts MailSyncOptions) ([]MailSyncMessage, error) {
+	if folderID <= 0 {
+		folderID = MailFolderInbox
+	}
+	var out []MailSyncMessage
+	skipped := 0
+	for page := 1; ; page++ {
+		batch, err := c.ResponseArray(ctx,
+			mailMessagesPath(MailMessagesFilter{Folder: folderID}, page, mailMessagesPageSize))
+		if err != nil {
+			return nil, fmt.Errorf("FetchMailFolder: %w", err)
+		}
+		if len(batch) == 0 {
+			break
+		}
+		for _, raw := range batch {
+			if skipped < opts.StartIndex {
+				skipped++
+				continue
+			}
+			msg, err := c.hydrateMailMessage(ctx, raw, opts)
+			if err != nil {
+				return nil, err
+			}
+			out = append(out, *msg)
+			if opts.Limit > 0 && len(out) >= opts.Limit {
+				return out, nil
+			}
+		}
+		if len(batch) < mailMessagesPageSize {
+			break
+		}
+	}
+	return out, nil
+}
+
+// hydrateMailMessage converts one raw API message into a MailSyncMessage,
+// fetching the full record when the list item does not carry the attachment
+// metadata, and downloading bodies when requested.
+func (c *Client) hydrateMailMessage(ctx context.Context, m map[string]any, opts MailSyncOptions) (*MailSyncMessage, error) {
+	msg := &MailSyncMessage{
+		ID:      Int64FromMap(m, "id"),
+		Folder:  int(Int64FromMap(m, "folder")),
+		Subject: stringFromMap(m, "subject"),
+		From:    stringFromMap(m, "from"),
+		IsNew:   boolFromMap(m, "isNew") == "true",
+	}
+	msg.Date = parseMailTime(stringFromMap(m, "date"))
+
+	atts, _ := m["attachments"].([]any)
+	hasFlag := boolFromMap(m, "hasAttachments") == "true"
+	if hasFlag && len(atts) == 0 {
+		// List items may omit the attachment array; pull the full record.
+		full, err := c.GetMailMessage(ctx, strconv.FormatInt(msg.ID, 10))
+		if err != nil {
+			return nil, fmt.Errorf("FetchMailFolder: hydrate message %d: %w", msg.ID, err)
+		}
+		atts, _ = full["attachments"].([]any)
+	}
+	for _, a := range atts {
+		am, ok := a.(map[string]any)
+		if !ok {
+			continue
+		}
+		att := MailSyncAttachment{
+			ID:   mailAttachmentID(am),
+			Name: stringFromMap(am, "fileName"),
+			Size: Int64FromMap(am, "size"),
+		}
+		if att.Name == "" {
+			att.Name = stringFromMap(am, "name")
+		}
+		if att.ID != "" {
+			msg.Attachments = append(msg.Attachments, att)
+		}
+	}
+	msg.HasAttachments = hasFlag || len(msg.Attachments) > 0
+
+	if opts.FetchBodies {
+		for i := range msg.Attachments {
+			body, err := c.DownloadMailAttachment(ctx, msg.Attachments[i].ID)
+			if err != nil {
+				return nil, fmt.Errorf("FetchMailFolder: message %d attachment %q: %w",
+					msg.ID, msg.Attachments[i].Name, err)
+			}
+			msg.Attachments[i].Body = body
+		}
+	}
+	return msg, nil
+}
+
+// mailAttachmentID extracts the download id from an attachment object.
+// OnlyOffice variants use "id", "fileId" or "attachmentId".
+func mailAttachmentID(am map[string]any) string {
+	for _, key := range []string{"id", "fileId", "attachmentId"} {
+		switch v := am[key].(type) {
+		case string:
+			if s := v; s != "" {
+				return s
+			}
+		case float64:
+			if n := int64(v); n != 0 {
+				return strconv.FormatInt(n, 10)
+			}
+		case int64:
+			if v != 0 {
+				return strconv.FormatInt(v, 10)
+			}
+		}
+	}
+	return ""
+}
+
+// parseMailTime accepts the OnlyOffice timestamp shapes seen in the wild:
+// RFC3339 (with any fractional digits) and second-precision local form.
+func parseMailTime(s string) time.Time {
+	if s == "" {
+		return time.Time{}
+	}
+	if t, err := time.Parse(time.RFC3339, s); err == nil {
+		return t
+	}
+	if t, err := time.Parse("2006-01-02T15:04:05", s); err == nil {
+		return t
+	}
+	return time.Time{}
+}