Implement skipped wslc E2E test

[AmelBawa-msft]

Summary of the Pull Request

Implement skipped wslc E2E test

Related:

• http://task.ms/62835763

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR re-enables and implements previously skipped WSLC end-to-end coverage by replacing placeholder/disabled tests with working implementations that exercise the real CLI behavior (stdin import, terminal detection, and --pull behavior against a private registry source).

Changes:

• Implement image import from stdin E2E by saving a tar archive and feeding it to wslc image import - ... via a file-backed stdin handle.
• Re-enable image save “stdout is a terminal” failure E2E by running wslc under a pseudoconsole and waiting for the localized error output.
• Re-enable build --pull E2E by standing up a local registry, tagging/pushing a base image to it, and validating digest resolution appears in verbose build progress.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/windows/wslc/e2e/WSLCE2EImageSaveTests.cpp	Replaces a skipped test with a pseudoconsole-backed interactive run validating “save to terminal” failure behavior.
test/windows/wslc/e2e/WSLCE2EImageImportTests.cpp	Implements the previously-not-implemented stdin import success path using a saved tarball as stdin.
test/windows/wslc/e2e/WSLCE2EImageBuildTests.cpp	Replaces an unstable skip with a local-registry-backed --pull test validating digest resolution in verbose output.
test/windows/wslc/e2e/WSLCE2EHelpers.h	Adds a shared helper declaration for waiting on pseudoconsole output substrings.
test/windows/wslc/e2e/WSLCE2EHelpers.cpp	Implements the pseudoconsole output wait helper and refactors tty-size waiting to reuse it.