Fix: Correctly apply Kubernetes manifests from remote URLs

NOTE: We had re-raise this PR due to a regression found in the original PR #4292 . The root cause was identified:

Some of the integration tests are likely passing manifest objects that do not have a 
source attribute at all (for example, a manifest that only uses the content attribute).
In these cases, manifest.source is null. The startswith() function cannot accept a null 
value; it requires a string. This causes Terraform to stop with an error.

This PR resolves a bug that caused Terraform to fail with an Invalid count argument error when applying a Kubernetes manifest from a remote URL. The issue occurred when the kubectl-apply module had a dependency on another resource i.e. GKE cluster, being created in the same run.

The root cause was a Terraform plan/apply lifecycle conflict. The module was trying to fetch the URL content after the cluster was created (in the apply phase), but other parts of the plan depended on that content before the apply could start.

This fix refactors the kubectl-apply module with the following strategy:

• The parent "controller" module now fetches all URL-based manifest content during the terraform plan phase.
• It then passes the raw text content not the URL to the submodule responsible for the apply.
• The submodule has been simplified to no longer handle URLs, completely resolving the timing conflict.

This change allows for the robust and predictable deployment of URL-based manifests.

How to Test and Verify

1. Configure a Blueprint: Use a blueprint that calls the workload-manager-install module (modules/management/kubectl-apply).

2. Use a URL Source: In the settings, disable the internal Kueue installation and provide the Kueue manifest via a URL in the apply_manifests list. This configuration specifically targets the fixed logic.

YAML

# Example blueprint settings:
settings:
 # Ensure internal install is disabled to avoid conflicts
 # kueue:
 #   install: true

 apply_manifests:
 - source: "https://raw.githubusercontent.com/GoogleCloudPlatform/cluster-toolkit/refs/heads/develop/modules/management/kubectl-apply/manifests/kueue-v0.11.4.yaml"
   # Flags needed for large manifests and pre-existing resources
   server_side_apply: true
   force_conflicts: true 

3. Deploy: Run the deployment command (e.g., ./gcluster deploy ...).

4. Expected Result:
   The terraform plan and terraform apply phases should complete successfully without any "Invalid count" or other planning errors.

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