test: add PostCleanup precheck

[hardcoretime]

Description

Add PostCleanup precheck to the e2e test framework and integrate it with the Config struct.

Changes include:

• Add IsCleanupNeeded field to Config struct in test/e2e/internal/config/config.go
• Add default value isCleanupNeeded: true in test/e2e/default_config.yaml
• Create new postcleanup precheck in test/e2e/internal/precheck/postcleanup.go that validates the POST_CLEANUP environment variable
• Add PrecheckPostCleanup label to test/e2e/internal/precheck/labels.go
• Refactor legacy test files to use conf.IsCleanupNeeded instead of the old config.IsCleanUpNeeded() function
• Refactor framework.go and suite_cvi.go to use GetConfig().IsCleanupNeeded
• Update test/e2e/README.md to document the new configuration option

Why do we need it, and what problem does it solve?

The e2e test framework previously used the config.IsCleanUpNeeded() function that directly read the POST_CLEANUP environment variable without considering the YAML configuration. This change:

1. Moves cleanup configuration to the Config struct, making it consistent with other configuration options
2. Adds a proper precheck that validates the POST_CLEANUP environment variable before running tests
3. Provides better documentation for users about how to control cleanup behavior

What is the expected result?

• Tests continue to work with the default cleanup behavior (enabled)
• Users can disable cleanup via POST_CLEANUP=no environment variable (takes precedence over YAML)
• Users can configure default cleanup behavior via isCleanupNeeded field in default_config.yaml
• All legacy tests and framework use the same configuration source

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: test
type: chore
summary: "Add PostCleanup precheck and IsCleanupNeeded config option for e2e tests."
impact_level: low