Four fixes for the Weixin/WeChat adapter, synthesized from the best
aspects of community PRs #8407, #8521, #8360, #7695, #8308, #8525,
#7531, #8144, #8251.
1. Streaming cursor (▉) stuck permanently — WeChat doesn't support
message editing, so the cursor appended during streaming can never
be removed. Add SUPPORTS_MESSAGE_EDITING = False to WeixinAdapter
and check it in gateway/run.py to use an empty cursor for non-edit
platforms. (Fixes#8307, #8326)
2. Media upload failures — two bugs in _send_file():
a) upload_full_url path used PUT (404 on WeChat CDN); now uses POST.
b) aes_key was base64(raw_bytes) but the iLink API expects
base64(hex_string); images showed as grey boxes. (Fixes#8352, #7529)
Also: unified both upload paths into _upload_ciphertext(), preferring
upload_full_url. Added send_video/send_voice methods and voice_item
media builder for audio/.silk files. Added video_md5 field.
3. Markdown links stripped — WeChat can't render [text](url), so
format_message() now converts them to 'text (url)' plaintext.
Code blocks are preserved. (Fixes#7617)
4. Blank message prevention — three guards:
a) _split_text_for_weixin_delivery('') returns [] not ['']
b) send() filters empty/whitespace chunks before _send_text_chunk
c) _send_message() raises ValueError for empty text as safety net
Community credit: joei4cm (#8407), lyonDan (#8521), SKFDJKLDG (#8360),
tomqiaozc (#7695), joshleeeeee (#8308), luoxiao6645(#8525),
longsizhuo (#7531), Astral-Yang (#8144), QingWei-Li (#8251).
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