[Backport v1.26] Add cache namespace options for DatadogCSIDriver controller

[dd-octo-sts]

Backport f41ef99 from #2934.

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What does this PR do?

Adds cache namespace configuration for the DatadogCSIDriver controller, consistent with all other controllers.

Introduces DD_CSIDRIVER_WATCH_NAMESPACE env var (falls back to WATCH_NAMESPACE), wires DatadogCSIDriverEnabled into config.CacheOptions(), and adds test coverage.

Motivation

The DatadogCSIDriver controller was the only controller missing its own cache namespace entry. Without it, the controller fell back to DefaultNamespaces (driven by DD_AGENT_WATCH_NAMESPACE/WATCH_NAMESPACE) instead of having its own DD_CSIDRIVER_WATCH_NAMESPACE override.

Additionally, DatadogCSIDriver is unique among controllers in that it creates and reconciles a native k8s DaemonSet, which must also be accessible from the cache. If the CSIDriver is deployed in a different namespace than the agent (e.g. DD_CSIDRIVER_WATCH_NAMESPACE=default, DD_AGENT_WATCH_NAMESPACE=system), the DaemonSet falls outside DefaultNamespaces and causes:

getting DaemonSet: unable to get: default/datadog-csi-driver-node-server because of unknown namespace for the cache

The fix explicitly adds DaemonSet to ByObject with the union of agent and CSIDriver namespaces. Using DefaultNamespaces instead would over-cache all other GVKs (Secrets, ConfigMaps, etc.) in the CSIDriver namespace unnecessarily.

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
• All commits are signed (see: signing commits)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 40.68%. Comparing base (e5cf461) to head (1da7286).

Files with missing lines	Patch %	Lines
cmd/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            v1.26    #2940      +/-   ##
==========================================
+ Coverage   40.67%   40.68%   +0.01%     
==========================================
  Files         321      321              
  Lines       28482    28492      +10     
==========================================
+ Hits        11584    11593       +9     
- Misses      16043    16044       +1     
  Partials      855      855              

Flag	Coverage Δ
unittests	40.68% <90.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/config/config.go	66.66% <100.00%> (+3.44%)	⬆️
cmd/main.go	6.64% <0.00%> (-0.02%)	⬇️

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