fix(weixin): reject nonzero send ret

[louiseboo]

Summary

• Treat a non-zero iLink ret as a failed WeChat text send even when errcode is zero.
• Include ret, errcode, and the server message in the raised error.
• Add a regression test for the observed sanitized response {ret: -2, errcode: 0, errmsg: "prepare failed"}.

Fixes #210

Why

_send_text() only checked errcode. iLink can reject message preparation with a non-zero ret and errcode=0, causing Raven callers to report a false successful delivery. _poll_once() already treats either field as authoritative failure status.

Verification

• Red: the new regression test failed with DID NOT RAISE before the implementation change.
• Green: uv run pytest tests/test_channels_weixin.py::test_send_text_rejects_nonzero_ret_even_when_errcode_is_zero -q → 1 passed.
• Relevant suite: uv run pytest tests/test_channels_weixin.py tests/test_channels_manager.py tests/test_channels_outlet.py -q → 71 passed.
• uv run ruff check raven/channels/adapters/weixin/channel.py tests/test_channels_weixin.py → clean.
• uv run ruff format --check raven/channels/adapters/weixin/channel.py tests/test_channels_weixin.py → clean.

Risk

Low. The success path (ret=0, errcode=0, or absent zero-default fields) is unchanged. The only behavior change is that a server-declared failure now propagates to the caller instead of being silently treated as success.

Rollback

Revert commit 2d8cd2f; no data migration or configuration change is involved.

Privacy

The issue and test use sanitized identifiers and content. No user tokens, chat IDs, local paths, or private messages are included.