fix(weixin): drop manual Content-Length header rejected by undici

[yikkuro]

What

Removes the manual Content-Length header from the WeChat (weixin) plugin's buildHeaders().

Why

The WeChat channel was completely broken: inbound messages went unanswered and the WeChat client showed "暂无法连接 OpenClaw". The gateway logged an endless stream of TypeError: fetch failed for getUpdates, sendMessage, and sendTyping.

Root cause: buildHeaders() set Content-Length manually on every request. The gateway's bundled undici forbids this and throws InvalidArgumentError: invalid content-length header (UND_ERR_INVALID_ARG), which fetch re-wraps as the opaque TypeError: fetch failed. Because buildHeaders() backs every API call, the entire channel failed.

The real cause was hidden — fetch failed never exposes err.cause in the logs, and the same request succeeds from a standalone Node process (top-level built-in fetch is more lenient than the gateway's undici). It was confirmed by temporarily wrapping globalThis.fetch in the gateway to capture err.cause.

Fix

Delete the manual Content-Length header (and the now-unused body param of buildHeaders). fetch/undici computes Content-Length from the string body automatically.

Verification

Applied to the live gateway plugin and restarted:

• Before: getUpdates/sendMessage/sendTyping failed every ~2s with fetch failed.
• After: monitor holds a stable long-poll (ESTABLISHED connection), zero fetch failed, and a real WeChat "Hello" round-trips and receives a reply (confirmed by the reporter).

Fixes #55