fix(webhook): forward response attachments to delivery platform

[LuneZ99]

Webhook-triggered agent responses currently send text through the configured cross-platform adapter, but attachment methods fall back to BasePlatformAdapter on the Webhook adapter. This produces Couldn’t deliver the file attachment even when the destination Feishu adapter supports native uploads.

This change forwards document, image, audio, and video response attachments through the configured delivery adapter while keeping destination thread metadata from deliver_extra.

Validation:

• ruff check .
• 42 webhook adapter/integration tests passed
• 76 Feishu tests passed
• full suite was started but stopped after unrelated missing optional dependency/baseline failures (acp, anthropic, executable-bit tests); 5,000+ tests had passed before stop.

[LuneZ99]

自审结论：未发现 important 级别以上问题。改动仅扩展现有 Webhook 跨平台分发入口，目标 adapter、chat_id 与 thread metadata 仍沿用原解析链；来源 webhook 的 reply metadata 不会误传到目标平台。文档、图片、音频、视频四条附件路径均有行为测试覆盖。Ruff、42 项 Webhook 测试和 76 项 Feishu 测试通过。