fix(kubit): add default resource overrides and correct namespaceOverride typo for app-instance

[evasilly]

Fix typo in namespaceOverride (was NamspaceOverride).
Add support for overriding Kubit resource requests and limits via .Values.kubit.resources.

What this PR does

• Fixes typo in namespaceOverride
• Adds support for overriding Kubit resource requests and limits via .Values.kubit.resources
• Adds default CPU/memory requests and limits for Kubit
• Prevents mapTrim requires a map when kubit: {} is empty in values.yaml

Why

Without these changes, the Kubit template fails rendering when resource overrides are not defined.
This ensures users can deploy with sensible defaults and adjust resource settings as needed.

How tested

• Rendered with helm template influxdb3-clustered -f values.yaml
  • Verified defaults applied correctly with empty kubit: {}
  • Verified override behavior works when custom resource values are provided
• Confirmed valid YAML output and Helm template success.

Checklist

• CHANGELOG.md updated (not applicable – no changelog in chart)
• Rebased/mergable
• Tests pass (helm template validation)
• Signed CLA