fix: classify withdrawn reply targets

[liangyu-suli]

Summary

• classify Feishu's code-less The message was withdrawn. response as target_revoked
• inspect the nested API response message before the generic HTTP 400 fallback
• preserve format_error classification for unrelated HTTP 400 responses

Why

When a bot replies to a user message that was withdrawn while the handler was running, Feishu may return HTTP 400 without a numeric platform code:

The message was withdrawn.

classifyError() previously read only the top-level error message and then classified every HTTP 400 as format_error. As a result, callers using isReplyTargetGone() could not fall back to a fresh, unanchored send.

Downstream report: zarazhangrui/lark-coding-agent-bridge#260
Downstream defensive fallback: zarazhangrui/lark-coding-agent-bridge#281

Validation

• pnpm test — 548 tests passed
• pnpm typecheck — passed
• pnpm build — passed
• pnpm lint — no errors; reports pre-existing warnings/info in unrelated files and a stale Biome schema version

[CLAassistant]

All committers have signed the CLA.