feat(commands): /test sends the media, /debug takes over the parse report

- `/test <url>` now runs the ordinary link pipeline and actually sends the
  media, but with the chat's post-send actions suppressed: no channel forward,
  no edit-before-forward prompt. It is the same code path as a normal link
  (same caption/format handling, link cache, retries, dead-letter
  notification), so "does this link work?" is answered by the send itself.
- `/debug <url>` keeps what `/test` used to do: fetch and reply with the HTML
  parse report, sending/caching/forwarding nothing.
- `urls::url_media` takes a `PostSend` mode (`FromChat` for the URL workers,
  `Suppressed` for `/test`); `build_send_task` maps it to the task's
  `edit_before_forward`/`forward_channel_id`. Notification ids stay set in both
  modes, so a queued retry still reports a dead-letter to the chat.
- `/test` rejects an unsupported URL with the same message the old parse-only
  command used (the URL flow would otherwise ignore it silently).
- Report builder renamed `test_parse_report` -> `debug_report` (with the cap
  constant), `parse_test_arg` -> `parse_arg_remainder` (now shared by both
  commands). README/README.en command tables and AGENTS.md updated; `/help`
  descriptions come from the enum.

Tests: +3 (normal flow still honours the chat's settings, `/test` sends with
them suppressed and keeps the cache entry, `build_send_task` mode mapping). The
suppression test was verified to fail when the mode is ignored.
fmt/clippy clean, 73 + 69 tests pass.

README.md

@@ -114,7 +114,8 @@ Telegram 只接受 443/80/88/8443 端口。
 | `/set_format <站点> <格式>` | 自定义某站点的 caption 格式。站点：`twitter` / `bsky` / `pixiv` / `misskey`。占位符：`{url}` `{author}` `{author_url}` `{title}` `{tags}` |
 | `/clear_cache [链接]` | 清空链接缓存（仅管理员）；带链接只清该条，否则清空全部 |
 | `/bot_dict` | 查看当前聊天状态（调试用；仅管理员） |
-| `/test <链接>` | 调试：只解析链接并返回解析结果（站点、标题、作者、标签、媒体列表），不发送任何媒体 |
+| `/test <链接>` | 解析链接并发送媒体；不转发到频道、不弹转发前编辑提示（仅发送） |
+| `/debug <链接>` | 调试：只解析链接并返回解析结果（站点、标题、作者、标签、媒体列表），不发送任何媒体 |
 
 链接处理仅限私聊；命令在任意聊天可用。