feat(bcs): fire system message on session file upload complete

[zh-bupt]

What

When a bot or human completes a session file upload, fire a GenericNotification system message to the other bots (via chat.inject) and the frontend ws, using the fixed text format:

{prefix} {uploader} 上传了一个文件 {fileName} ({fileId}，{readableSize})，下载链接：{url}

• prefix is Bot for a bot uploader, 用户 for a human.
• Example: Bot 测试Agent 上传了一个文件 readme.md (THISISFILEID，12 KB)，下载链接：https://bcn.alipay.com/sessions/SESSION_ID/files/THISISFILEID/content

How

Reuses the existing SystemMessageEvent::GenericNotification path — no new event kind, producer, or bootstrap registration. All changes live in one file: crates/adapters/http/bcs-http/src/routes/session_files.rs.

• complete_upload captures the session and, on success, calls a new best-effort notify_file_uploaded helper. notify is best-effort and only runs on the Ok path, so it never fails the upload response.
• Recipients = the session's bot participants excluding the uploader. If there are no other bots, the notification is skipped entirely — GenericNotificationMessageProducer falls back to all bot participants on empty receivers, which would self-inject the uploader.
• Download link uses bcs_endpoint (from BcsConfig.bcs_endpoint) with an http://{bind}:{port} fallback and a URL-encoded session id. The link is permanent and resolves for all recipients (bot bearers / human session members).
• Uploader name: bot via registry.get(..)->capabilities.name (fallback id); human via nick_name (fallback staff_no).

New private helpers in the route module: human_readable_size, file_download_url, file_upload_receivers, uploader_display_name; plus a RecordingSystemMessage test spy wired into the in-module TestApp fixture.

Diff scope

session_files.rs only — +405 / −7.

Testing

cargo test -p bcs-http --lib → 74 passed / 0 failed; cargo build -p bcs-http warning-free.

Coverage:

• Bot-uploader integration test (HTTP through the router) — asserts the exact message text, group_id, and receivers == [other-bot].
• Failure test — a failed complete-upload (404) fires no notify.
• Guard unit test — uploader is the only bot → no notify (no self-inject, no frontend broadcast).
• Human-uploader unit test — produces a 用户 ... message delivered to all bots.