fix(messaging): suppress NO_REPLY silent token on WeChat outbound

[vyrnsynx]

Summary

• When a turn's final reply is the OpenClaw silent token NO_REPLY / no_reply, the WeChat outbound path was delivering the literal string as a real message (textLen=8).
• Gate outbound text with the host isSilentReplyText helper (via a thin isSilentOutboundText wrapper) so exact silent tokens are skipped before sendMessage / deliver / channel outbound.
• If media is present alongside a silent token, strip the token text and still deliver the media.

Fixes #293

Changes

File	Change
src/messaging/silent-reply.ts	Wrapper around SDK isSilentReplyText
src/messaging/send.ts	Defense-in-depth skip in sendMessageWeixin
src/messaging/process-message.ts	Skip/strip in reply deliver callback
src/channel.ts	Skip/strip in sendWeixinOutbound / sendMedia
src/messaging/silent-reply.test.ts / send.test.ts	Unit coverage for suppress + non-suppress cases

Test plan

• npx vitest run --coverage=false src/messaging/silent-reply.test.ts src/messaging/send.test.ts (29 passed)
• npx vitest run --coverage=false src/messaging (273 passed)
• oxfmt --check on touched files
• Live: heartbeat/background turn that ends with NO_REPLY no longer appears in WeChat chat
• Live: normal text replies still deliver
• Live: media + silent token still delivers media without the NO_REPLY caption