feat(xtest): Published artifacts for lts/latest versions

[pflynn-virtru]

Summary

Currently, xtest builds all SDK versions from source. For lts and latest (released versions), we could alternatively consume published artifacts (npm packages, Maven JARs).

Potential Benefits

1. Fidelity: Tests the exact artifact users download, catching packaging issues (missing files, bad dependencies) that source builds miss
2. Performance: Skips compilation step

Current Constraints

The CLI wrappers in xtest/sdk/{js,java,go} are designed to build from source. Implementing this would require "hybrid mode" support.

Considerations

• xtest's primary purpose is cross-SDK compatibility testing, not release artifact verification
• We've already added dependency caching, which mitigates performance concerns
• A separate "release smoke test" workflow in each SDK repo might be a cleaner approach
• Adds complexity to an already complex build system

Decision

Documenting for future consideration. Lower priority than other improvements.

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Generated during Issue #377 discussion