check.milter: fail_open does not cover a milter session-dial failure

阅读 internal/check/milter/milter.go，尤其是 CheckStateForMsg 和现有的 ioError 处理。检查 PR #865 中提到的回归测试，并验证启用 fail_open 时无法访问的 milter 会未经检查地接受消息，同时测试通过。

Summary

check.milter's fail_open directive is documented to let a message through when the milter can't be consulted, but it silently doesn't cover one specific failure mode: the milter being completely unreachable when maddy tries to establish a session.

Root cause

CheckStateForMsg (internal/check/milter/milter.go) dials the milter backend via c.cl.Session() and returns any error unconditionally. This happens before a *state exists, so the failure never reaches ioError() — the only place fail_open is actually checked. Result: with fail_open set, a message is still hard-rejected (a real SMTP-level error handed back to the original sender) if the milter is simply down — exactly the case fail_open exists to guard against.

Reproduction

Configure check.milter with fail_open: true pointing at a milter address that's genuinely unreachable (e.g. nothing listening on that port), then send a message through. Expected: message accepted unchecked, per fail_open. Actual: a hard SMTP-level rejection.

Fix

I've prepared and tested a fix — handles the dial failure the same way ioError() already handles a later I/O failure — and opened it as #865, with a regression test that fails against unpatched code and passes with the fix.

Apologies for the PR arriving before this issue — I realize CONTRIBUTING.md's process is to raise the issue first. Filing this now so there's a proper place to discuss the bug alongside the PR.