feat(kurl-migration): kurl migration api foundation

[diamonwiggins]

What this PR does / why we need it:

This PR implements the foundation for the kURL to Embedded Cluster migration by starting the API server in migration mode when a kURL cluster is detected during upgrade. This allows the migration UI to be accessible and validates that the skeleton migration API endpoints are functional.

Key changes:

• Modified upgrade.go to start the API server in migration mode when ENABLE_V3=1 and a kURL cluster is detected
• Added runMigrationAPI() function that generates TLS certificates and bcrypt password hash at runtime
• Added dryrun test TestUpgradeKURLMigration to validate the migration API endpoints
• Test verifies that POST /api/linux/kurl-migration/start and GET /api/linux/kurl-migration/status return expected skeleton responses

Which issue(s) this PR fixes:

Relates to the kURL Migration Foundation work (Shortcut story TBD)

Does this PR require a test?

Yes, added TestUpgradeKURLMigration in tests/dryrun/upgrade_kurl_migration_test.go that validates:

• API server starts correctly in migration mode
• Authentication works with bcrypt password
• Migration endpoints return expected skeleton responses
• Migration state transitions from InProgress to Failed with "migration phase not implemented" error

Does this PR require a release note?

NONE

Does this PR require documentation?

NONE

[github-actions]

This PR has been released (on staging) and is available for download with a embedded-cluster-smoke-test-staging-app license ID.

Online Installer:

curl "https://staging.replicated.app/embedded/embedded-cluster-smoke-test-staging-app/ci/appver-dev-83e9833" -H "Authorization: $EC_SMOKE_TEST_LICENSE_ID" -o embedded-cluster-smoke-test-staging-app-ci.tgz

Airgap Installer (may take a few minutes before the airgap bundle is built):

curl "https://staging.replicated.app/embedded/embedded-cluster-smoke-test-staging-app/ci-airgap/appver-dev-83e9833?airgap=true" -H "Authorization: $EC_SMOKE_TEST_LICENSE_ID" -o embedded-cluster-smoke-test-staging-app-ci.tgz

Happy debugging!