feat(feishu): enhance channel with markdown cards, media, mentions, and editing

Upgrade the Feishu channel from basic text-only to full feature parity with
Telegram/Discord: interactive card messages with markdown rendering, message
editing (MessageEditor), placeholder messages (PlaceholderCapable), emoji
reactions (ReactionCapable), and inbound/outbound media support (MediaSender).

Also add @mention detection with lazy bot open_id discovery, group trigger
filtering with mention awareness, and multi-type inbound message parsing
(text, post, image, file, audio, video).

pkg/channels/feishu/common.go

@@ -1,5 +1,13 @@
 package feishu
 
+import (
+	"encoding/json"
+	"regexp"
+	"strings"
+
+	larkim "github.com/larksuite/oapi-sdk-go/v3/service/im/v1"
+)
+
 // stringValue safely dereferences a *string pointer.
 func stringValue(v *string) string {
 	if v == nil {
@@ -7,3 +15,78 @@ func stringValue(v *string) string {
 	}
 	return *v
 }
+
+// buildMarkdownCard builds a Feishu Interactive Card JSON 2.0 string with markdown content.
+// JSON 2.0 cards support full CommonMark standard markdown syntax.
+func buildMarkdownCard(content string) (string, error) {
+	card := map[string]any{
+		"schema": "2.0",
+		"body": map[string]any{
+			"elements": []map[string]any{
+				{
+					"tag":     "markdown",
+					"content": content,
+				},
+			},
+		},
+	}
+	data, err := json.Marshal(card)
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}
+
+// extractImageKey extracts the image_key from a Feishu image message content JSON.
+// Format: {"image_key": "img_xxx"}
+func extractImageKey(content string) string {
+	var payload struct {
+		ImageKey string `json:"image_key"`
+	}
+	if err := json.Unmarshal([]byte(content), &payload); err != nil {
+		return ""
+	}
+	return payload.ImageKey
+}
+
+// extractFileKey extracts the file_key from a Feishu file/audio message content JSON.
+// Format: {"file_key": "file_xxx", "file_name": "...", ...}
+func extractFileKey(content string) string {
+	var payload struct {
+		FileKey string `json:"file_key"`
+	}
+	if err := json.Unmarshal([]byte(content), &payload); err != nil {
+		return ""
+	}
+	return payload.FileKey
+}
+
+// extractFileName extracts the file_name from a Feishu file message content JSON.
+func extractFileName(content string) string {
+	var payload struct {
+		FileName string `json:"file_name"`
+	}
+	if err := json.Unmarshal([]byte(content), &payload); err != nil {
+		return ""
+	}
+	return payload.FileName
+}
+
+// mentionPlaceholderRegex matches @_user_N placeholders inserted by Feishu for mentions.
+var mentionPlaceholderRegex = regexp.MustCompile(`@_user_\d+`)
+
+// stripMentionPlaceholders removes @_user_N placeholders from the text content.
+// These are inserted by Feishu when users @mention someone in a message.
+func stripMentionPlaceholders(content string, mentions []*larkim.MentionEvent) string {
+	if len(mentions) == 0 {
+		return content
+	}
+	for _, m := range mentions {
+		if m.Key != nil && *m.Key != "" {
+			content = strings.ReplaceAll(content, *m.Key, "")
+		}
+	}
+	// Also clean up any remaining @_user_N patterns
+	content = mentionPlaceholderRegex.ReplaceAllString(content, "")
+	return strings.TrimSpace(content)
+}