chore(datadog): sync values.schema.json with values.yaml to fix helm lint

[TPAlves]

Title: chore(datadog): sync values.schema.json with values.yaml to fix helm lint

Summary
Synchronizes charts/datadog/values.schema.json with charts/datadog/values.yaml so Helm's JSON schema validation no longer reports invalid/missing top-level properties. Also permits registry to be null to match current values.yaml.

What this PR does / why we need it:

• Adds top-level properties to values.schema.json that are present in values.yaml (e.g. clusterAgent, existingClusterAgent, fips, kube-state-metrics, agents, providers, registry, remoteConfiguration, clusterChecksRunner, datadog-crds, otelAgentGateway, targetSystem, useFIPSAgent, commonLabels, nameOverride, fullnameOverride).
• Allows "registry" to be null (type: ["object","null"]) to match existing default in values.yaml.
• Prevents helm lint errors caused by schema/value mismatch without changing chart runtime behavior.

Which issue this PR fixes

• fixes #

Special notes for your reviewer:

• This is a sync fix to stop validation failures. Types for the added top-level properties are intentionally permissive (object / additionalProperties) to avoid breaking behavior; I recommend follow-up PRs to tighten schema definitions per-property.
• Chart dependencies (datadog-crds, kube-state-metrics, datadog-csi-driver) were not vendored — lint now passes with a single warning about missing dependencies. If maintainers prefer vendoring, I can add them in a follow-up or update CI to run helm dependency build.
• Commands run:
  • helm lint ./charts/datadog -> passes with 1 warning (missing dependencies)
  • (optional) helm install --dry-run --debug may still require running helm dependency build due to the vendored-deps warning.

[WARNING] /home/thiago/helm-charts/charts/datadog: chart directory is missing these dependencies: datadog-crds,kube-state-metrics,datadog-csi-driver

1 chart(s) linted, 0 chart(s) failed

Checklist

• Chart Version semver bump label 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)

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