test: integrate precreated CVI into precheck system

[hardcoretime]

Description

This PR integrates precreated ClusterVirtualImages (CVIs) into the precheck system for e2e tests.

Changes:

1. New precheck implementation (internal/precheck/precreatedcvi.go):

   • Added PrecheckPrecreatedCVI as a common precheck that creates/verifies precreated CVIs before tests
   • Validates PRECREATED_CVI_CLEANUP env variable
   • Implements CleanupPrecreatedCVIs() function for cleanup in SynchronizedAfterSuite

2. Config integration:

   • Added IsPrecreatedCVICleanupNeeded field to Config struct
   • Added support for PRECREATED_CVI_CLEANUP env variable with config priority over yaml
   • Added proper comments explaining the difference between post cleanup and precreated CVI cleanup

3. Code organization:

   • Moved image URL constants from const.go to precreated_cvi.go for better cohesion
   • Removed legacy cleanup functions from config/cleanup.go and legacy/legacy.go
   • Removed unused suite_cvi.go and internal/precreatedcvi package

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

This refactoring consolidates all precreated CVI handling into the precheck system, making it consistent with other prechecks like PostCleanupPrecheck.

What is the expected result?

• Precreated CVIs are automatically created/verified before tests run (via common precheck)
• Cleanup behavior controlled via PRECREATED_CVI_CLEANUP env var or isPrecreatedCVICleanupNeeded config field
• By default, CVIs persist between runs for faster test execution
• Set PRECREATED_CVI_CLEANUP=yes to delete CVIs after the suite

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: "Integrate precreated CVI lifecycle into precheck system for e2e tests."
impact_level: low

The base branch was changed.

[deckhouse-BOaTswain]

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