WIP feat(source): metric and event for invalid endpoints

[ivankatliarchuk]

WHY

Invalid endpoints — misconfigured CNAME self-references, malformed MX/SRV records, unsupported alias types — are silently dropped by the dedup source with only a log
warning. In production clusters this creates a blind spot: there is no way to alert on broken DNS configurations, measure how many exist, or trace which Kubernetes
resource is responsible. Operators discover the problem only by grepping logs or noticing that a DNS record never appears.

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WHAT

• Adds external_dns_source_invalid_endpoints{record_type, source_type} gauge — resets and refills each reconcile cycle, so it directly answers "how many broken endpoints
  exist right now" per record type and source. source_type is populated from RefObject.Source when available, falls back to "unknown" until source implementations provide
  RefObject.
• Emits a Kubernetes Event (action=FailedSync, reason=RecordError) per rejected endpoint, gated by the existing --emit-events=RecordError flag, so no extra configuration
  is needed.
• Extracts buildEventEmitter from buildController in execute.go so the emitter is constructed before the source pipeline and shared with the dedup layer — no duplication.

More

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

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