Invalidate the TestImageRegistry cache after running prune --all

[Blue (OneBlue)]

Summary of the Pull Request

This change solves an out-of-date test registry cache issue which can cause the following test failure:

StartGroup: WSLCE2ETests::WSLCE2EContainerRunTests::WSLCE2E_Container_Run_PortMultipleMappings
Error: Verify: AreEqual(*expected.Stderr, *Stderr) - Values (, Image 'python:3.12-alpine' not found, pulling
3.12-alpine: Pulling from library/python
55afa1ecc21d: Already exists
0021712cb493: Already exists
7eec6ed36766: Already exists
5086b6265f8c: Already exists
Digest: sha256:d09d15e60962ca365d1cd544a48773bac9d33f2fb1b00f2aa0deec78ade7dc31
Status: Downloaded newer image for python:3.12-alpine
) [File: C:/Users/piboulay/repos/wsl/test/windows/wslc/e2e/WSLCExecutor.cpp, Function: WSLCE2ETests::WSLCExecutionResult::Verify, Line: 91]
EndGroup: WSLCE2ETests::WSLCE2EContainerRunTests::WSLCE2E_Container_Run_PortMultipleMappings [Failed

That's because wslc prune --all can get rid of the python image, which causes a pull in later tests, causing test failures

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 updates the WSLC E2E test infrastructure to prevent stale TestImageRegistry cache state after wslc image prune --all, avoiding later tests incorrectly assuming images are still present (leading to unexpected pulls and stderr/output mismatches).

Changes:

• Added TestImageRegistry::InvalidateSession() to discard cached per-session image state after bulk-removal operations.
• Updated the WSLCE2E_Image_Prune_AllFlag test cleanup to invalidate the cache and restore the baseline Debian test image.
• Implemented cache invalidation by removing cached loaded-image keys and clearing the “seeded” marker for the session.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/windows/wslc/e2e/WSLCE2EImagePruneTests.cpp	Ensures image prune --all test cleanup invalidates cache and restores baseline image to avoid cross-test contamination.
test/windows/wslc/e2e/TestImageRegistry.h	Adds InvalidateSession() API to explicitly drop cached image state for a session.
test/windows/wslc/e2e/TestImageRegistry.cpp	Implements InvalidateSession() by clearing cached loaded images and the seeded-session marker.

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