[CONTP-1612] support enabling untaint controller in datadog chart

[adel121]

DO NOT MERGE YET

What this PR does / why we need it:

Which issue this PR fixes

For Helm-managed clusters (agent installed via charts/datadog, no operator managing the agent), users need to opt the agent into tolerating the startup taint so it can schedule on tainted nodes. The untaint controller's pod label selector already works for Helm-managed agents — charts/datadog uses agent.datadoghq.com/component: agent on the node agent DaemonSet — so no label change is needed, only the toleration.

In this PR, we:

• Add operator.untaintController.enabled value (default false) to values.yaml
• Inject the toleration into the agent DaemonSet template when enabled, and activate the untaint controller feature in the operator.
• Add Helm unit test verifying the toleration appears in the rendered DaemonSet

Testing

Added unit test

Manual testing:

Deploy the agent using helm chart with the added option enabled. Ensure the toleration is added properly:

Deployed agent with appended at the end of values.yml:

operator:
  untaintController:
    enabled: true

Checked that the toleration was properly added:

      tolerations:
      - effect: NoSchedule
        key: agent.datadoghq.com/not-ready
        operator: Equal
        value: presence

Special notes for your reviewer:

The CSI driver may also need a similar change, but since CSI is usually used as a subchart (dependency of the agent chart), and the user can already set tolerations manually on the CSI by editing datadog_csi_driver section in datadog values.yaml, I am keeping it for a future PR to asses if it is worth adding a separate config option for CSI or not.

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• All commits are signed and show as "Verified" on GitHub (see: signing commits)
• Chart Version semver bump label has been added (use <chartName>/minor-version, <chartName>/patch-version, or <chartName>/no-version-bump)
• For datadog or datadog-operator chart or value changes, update the test baselines (run: make update-test-baselines)
• For datadog chart changes, received ✅ from a member of your team

GitHub CI takes care of the below, but are still required:

• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md

DO NOT MERGE YET