[NO MERGE] feat: add jenkins agent selector logic to use env based agent

[gunnertwin]

Jira link

DTSPO-30107

Change description

This updates cnp-jenkins-library to run the standard app pipeline on environment-specific Jenkins agents and managed identities instead of the shared Jenkins agent identity.

The main app pipeline now selects a primary ubuntu- agent at startup, so checkout, build, static checks, container build, ACR promotion, Helm publish, and normal branch orchestration run on the relevant environment agent. Stages that target a different environment explicitly switch to that environment’s agent, for example preview AKS deploy/tests use ubuntu-preview, prod Terraform uses ubuntu-prod, and PTL Flux/Jenkins identity lookups use ubuntu-ptl.

This also updates Azure/AKS/ACR/DNS helpers so Azure CLI commands resolve the correct AZURE_CONFIG_DIR for the current environment agent, and keeps the existing shared-style paths only for backwards-compatible/custom paths where no product/environment is supplied.

Key changes:

• Added AgentSelector and withEnvironmentAgent helpers for environment-to-agent selection.
• Moved AKS deploy, smoke/functional tests, Helm uninstall, Terraform plan/apply, ACR build/promotion, Helm publish, and Flux reconcile onto environment-specific agents.
• Updated withSubscription so Terraform setup runs on the target env agent, while Jenkins/PTL metadata lookup runs on the PTL agent.
• Updated ACR, Helm, Kubectl, and private DNS helpers to use env-specific Azure config where appropriate.
• Added tests for agent selection, Azure config resolution, ACR/Helm/Kubectl/DNS behaviour, and withSubscription env/PTL switching.

Testing done

Automated:

• Ran ./gradlew test
• Result: 299 passed, 0 failed

Manual Jenkins validation:

• CFT Plum PR pipeline completed successfully using:
  • primary ubuntu-aat
  • preview deploy/test/uninstall on ubuntu-preview
  • no shared .azure-jenkins usage observed
• SDS Toffee master pipeline completed successfully using:
  • primary ubuntu-stg
  • stg infra/deploy/test on ubuntu-stg
  • prod Terraform plan/apply on ubuntu-prod
  • PTL metadata/Flux reconcile on ubuntu-ptl
  • no shared .azure-jenkins usage observed

Validation also confirmed:

• Terraform received non-empty TF_VAR_mgmt_subscription_id and TF_VAR_jenkins_AAD_objectId.
• Environment MIs could access required Terraform state storage keys for the tested stg/prod flows.
• Preview/stg AKS deploy, smoke, functional, DNS, and Helm uninstall paths worked end-to-end.

Security Vulnerability Assessment

CVE Suppression: Are there any CVEs present in the codebase, either newly introduced or pre-existing, that are being intentionally suppressed or ignored by this commit?

• Yes
• No