When WeCom returns errcode=40001 (invalid credential) or 42001 (token
expired), send() was returning a failure without evicting the bad token
from _access_tokens. All subsequent sends then kept using the same
invalid cached token until its TTL naturally expired (~7200s).
Fix: on the first token-rejection errcode, evict the cache entry and
retry once with a freshly fetched token. Non-token errcodes fail
immediately as before. If the refreshed token also fails, the error
is returned without looping further.
Adds four regression tests covering: successful retry on 40001,
successful retry on 42001, no retry on unrelated errcode, and clean
failure when the refresh does not help.
21db250034