docs: add webhook delivery semantics, retry behavior, and self-diagnosis guide

[rcsh1]

Intent

Ticket 1178598175370948986 (提币交易状态在Submitted没有收到webhook推送): There is no transaction-status-to-webhook trigger matrix, and no documentation on delivery timing, retry behavior, or SLA expectations. Document the full status-to-webhook trigger matrix and add a delivery semantics section.'

Summary

Improve the webhook documentation to help integrators self-diagnose missing webhook notifications (Ticket: 提币交易状态在 Submitted 没有收到 webhook 推送). Six files updated (EN + CN pairs).

Changed files

• v2/guides/webhooks-callbacks/set-up-endpoint.mdx — added ##### Retry behavior and ##### Delivery guarantees subsections
• v2_cn/guides/webhooks-callbacks/set-up-endpoint.mdx — CN mirror (##### 重试行为, ##### 投递保证)
• v2/guides/webhooks-callbacks/introduction.mdx — added ## Delivery semantics section and ### No webhook received? Self-diagnosis checklist
• v2_cn/guides/webhooks-callbacks/introduction.mdx — CN mirror (## 投递机制, ### 未收到 Webhook 事件？自助排查)
• v2/guides/webhooks-callbacks/webhook-event-type.mdx — added cross-link to transaction status reference
• v2_cn/guides/webhooks-callbacks/webhook-event-type.mdx — CN mirror cross-link

All edits restate facts already documented (2s timeout, 10 attempts, 200/201 success, ordering not guaranteed, duplicate delivery possible). No new timing, SLA, or dispatch-behavior claims were asserted.