Harden DNS CAA and TXT verification

Extract shared DNS checks into dnsclient and fail closed on
truncated or non-success CAA responses. Climb past eTLD+1,
validate RFC 8659 issue-value syntax, and map NXDOMAIN TXT
lookups to the pending-verification path.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/dnsclient/errors.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package dnsclient
+
+import "errors"
+
+var (
+	// ErrCAADenied means a CAA policy at the hostname or a parent domain
+	// forbids issuance by the requested certificate authority.
+	ErrCAADenied = errors.New("caa records do not permit issuance")
+
+	// ErrTXTNotFound means no TXT record exists at the queried domain name.
+	ErrTXTNotFound = errors.New("domain TXT record not found")
+
+	// ErrTXTMismatch means TXT records exist at the domain but none match the
+	// expected verification value.
+	ErrTXTMismatch = errors.New("domain TXT record mismatch")
+)