fix(mx): keep null MX and strip trailing dot

[mloiseleur]

What does it do ?

NewEndpointWithTTL trimmed the trailing dot off the whole MX target, turning the null MX
0 . (RFC 7505) into 0 , which no longer parses as an MX record.

source/crd.go passed MX targets through unchecked while every other source normalizes them
via NewEndpointWithTTL, so a DNSEndpoint with 10 mail.example.com. diffed on every
reconcile — on any provider.

It now normalizes instead, leaving unparseable targets and the null MX alone.

Motivation

Follow-up to #6660, split out per review.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

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[coveralls]

Coverage Report for CI Build 33159371836

Coverage increased (+0.01%) to 82.083%

Details

• Coverage increased (+0.01%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 14 coverage regressions across 3 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

14 previously-covered lines in 3 files lost coverage.

File	Lines Losing Coverage	Coverage
cloudflare/cloudflare_batch.go	8	96.28%
crd.go	4	89.61%
cloudflare/cloudflare.go	2	93.31%

---

Coverage Stats

Relevant Lines:	21349
Covered Lines:	17524
Line Coverage:	82.08%
Coverage Strength:	1465.38 hits per line

---

💛 - Coveralls

[mloiseleur]

Verified end to end against a real PowerDNS in Docker with a kind cluster and the CRD source, comparing master, the previous head of this branch, and the current one.

DNSEndpoint used:

spec:
  endpoints:
    - dnsName: example.com
      recordType: MX
      recordTTL: 300
      targets: ["10 mail.example.com.", "010  backup.example.com"]
    - dnsName: nomail.example.com
      recordType: MX
      recordTTL: 300
      targets: ["0 ."]

master:

CREATE: example.com 300 IN MX  010  backup.example.com;10 mail.example.com. []
fatal: unable to patch zone: Record example.com./MX '010  backup.example.com.':
       Not in expected format (parsed as '10 backup.example.com.')

Previous head of this branch — trailing dot fixed, still fatal:

CREATE: example.com 300 IN MX  010  backup.example.com;10 mail.example.com []
fatal: unable to patch zone: Record example.com./MX '010  backup.example.com.': Not in expected format

Current head:

run 1: CREATE: example.com 300 IN MX  10 backup.example.com;10 mail.example.com []
       CREATE: nomail.example.com 300 IN MX  0 . []
run 2: All records are already up to date
run 3: All records are already up to date
run 4: All records are already up to date

dig +short @127.0.0.1 -p 1053 example.com MX        -> 10 backup.example.com. / 10 mail.example.com.
dig +short @127.0.0.1 -p 1053 nomail.example.com MX -> 0 .

PowerDNS rejects a non-canonical MX target server-side, so the whole zone patch fails and the ownership TXT records and the null MX never get written either — not only reconcile churn.

Setup follows docs/tutorials/pdns.md, with --source=crd and --managed-record-types repeated once per type (it is a kingpin Strings(); a single comma-joined value parses as one element and silently filters everything out).

[ivankatliarchuk]

/lgtm