fix(dingtalk): honor mention-only groups and strip leading mentions

[Alix-007]

Summary

Fix a confirmed DingTalk no-response path behind issue #1895.

Changes

• use DingTalk callback isInAtList when evaluating group_trigger.mention_only
• strip leading @... tokens before routing and command handling so mentioned commands like @bot /help reach the built-in command path as /help
• fall back to senderId / conversationId when senderStaffId is empty for direct-message routing
• add focused tests for mention-only group delivery, mention stripping, and direct-message ID fallback

Validation

• go test ./pkg/channels/dingtalk
• go test ./pkg/commands
• go test ./pkg/channels

Issue tracking

• Partial progress on [BUG] Channel does not works #1895
• This PR intentionally does not claim the Feishu side of the report.

[Alix-007]

@alexhoshina CI is green on the current head. This PR only addresses the DingTalk side of #1895 and intentionally keeps the Feishu side open. A maintainer review would be appreciated when convenient.