Multiple receivers hide other errors in configuration

What did you do?

 ./amtool check-config config/testdata/conf.multi-bad-receivers.yml
Checking 'config/testdata/conf.multi-bad-receivers.yml'  FAILED: no global SMTP from set

What did you expect to see?

Errors from all three receivers.

What did you see instead? Under which circumstances?

Only a single error.

System information

No response

Alertmanager version

Alertmanager configuration file

global:
  resolve_timeout: 5m

receivers:
  # RECEIVER 1: Perfectly valid
  - name: 'team-alpha-webhook'
    webhook_configs:
      - url: 'https://webhook.alpha.com/alerts'

  # RECEIVER 2: Broken Email Config (The "Independent Loading" Issue)
  - name: 'team-beta-email'
    email_configs:
      - to: 'beta-team@example.com'
        # ERROR A: Mandatory field 'from' is missing (if not in global)
        # ERROR B: Invalid smarthost port
        smarthost: 'smtp.example.com:not-a-number' 

  # RECEIVER 3: Broken PagerDuty Config (The "Early Return" Issue)
  - name: 'team-gamma-pagerduty'
    pagerduty_configs:
      - service_key: '' # ERROR C: Required field empty
        # If the code returns early at ERROR C, it never sees ERROR D below
        routing_key: 'invalid-key-format-123' # ERROR D: Validation logic

route:
  group_by: ['alertname']
  receiver: 'team-alpha-webhook'
  routes:
    - match:
        team: 'beta'
      receiver: 'team-beta-email'

Prometheus version

Prometheus configuration file

Logs

Start by running ./amtool check-config config/testdata/conf.multi-bad-receivers.yml and inspect how configuration validation processes the receivers in that fixture. Trace the validation flow for the email and PagerDuty configurations, then add or update coverage so the command reports errors from all invalid receivers rather than stopping after one; done means the expected errors are visible together.