Add validations to DatadogMonitorSpec

[shuheiktgw]

What does this PR do?

This is the next iteration of this PR. I realized that we don’t actually need fancy CEL validations for most basic validations. So I updated the PR to use the simple CRD validations.

Motivation

Currently, the Datadog Operator does not implement validating webhooks and validates inputs during reconciliation. I assume this is to simplify the operator setup, but from a user experience perspective, it’s not ideal since users can only discover issues after applying their manifests.

Additional Notes

• Use validation rules to validate DatadogMonitorSpec #1692
• https://book.kubebuilder.io/reference/markers/crd-validation

Minimum Agent Versions

N/A

Describe your test plan

I've tested it locally using a Kind cluster with Kubernetes 1.32.

$ cat monitor.yaml
apiVersion: datadoghq.com/v1alpha1
kind: DatadogMonitor
metadata:
  name: datadog-monitor-test
spec:
  # query: ""
  type: "some alert"
  name: ""
  # message: "We are running out of disk space!"
  tags:
    - "test:datadog"

$ k apply -f monitor.yaml
The DatadogMonitor "datadog-monitor-test" is invalid:
* spec.name: Invalid value: "": spec.name in body should be at least 1 chars long
* spec.type: Unsupported value: "some alert": supported values: "metric alert", "query alert", "service check", "event alert", "log alert", "process alert", "rum alert", "trace-analytics alert", "slo alert", "event-v2 alert", "audit alert", "composite"
* spec.message: Required value
* spec.query: Required value

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.49%. Comparing base (53c0d8c) to head (568adb3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1694   +/-   ##
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  Coverage   49.49%   49.49%           
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  Files         218      218           
  Lines       21244    21244           
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  Hits        10515    10515           
  Misses      10182    10182           
  Partials      547      547           

Flag	Coverage Δ
unittests	49.49% <ø> (ø)

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