fix: download only required AutoTest fixtures

[wenyt (wenytang-ms)]

Summary

• Download only the vscode-java test project required by each AutoTest plan, using a depth-1 blob-filtered fetch and non-cone sparse checkout.
• Skip upstream downloads for plans using local fixtures (including Java 27), and remove the unused eclipse.jdt.ls clone.
• Retry both fetch and checkout failures up to three times, with explicit warnings and a final failure; never overwrite an existing sibling clone.
• Update the UI-test documentation and add regression coverage for all plan mappings, retries, and existing-clone protection.

Context

Run https://github.com/microsoft/vscode-java-pack/actions/runs/38349752918 failed before running the Windows java27-api case because an unnecessary vscode-java clone encountered Recv failure: Connection was reset.

This PR is limited to download optimization. Aggregate summary handling is intentionally unchanged.

Validation

• node --test tests/autotest-fixtures.test.js: all 6 tests passed, including fixture selection for every current YAML plan.
• Parsed the workflow YAML and checked helper wiring.
• Real GitHub partial-fetch/sparse-checkout smoke test: required simple-app/src/app/App.java present, unrelated root package.json absent, clean worktree, approximately 16 KiB of Git pack data.
• git diff --check.