refactor: reorganize e2e tests around features

[rmanibus]

Summary

• replace the monolithic E2E matrix scenario with a shared harness and matrix runner
• split behavior coverage into focused feature tests plus a smaller compatibility smoke matrix
• preserve the PR fix: resolve CI failures from dependency bump #172 race-condition fix by keeping backend metadata separate from factory invocation

Testing

• GOCACHE=$(mktemp -d /tmp/cloudstic-e2e-gocache-XXXXXX) go test ./e2e -count=1

Notes

• reviewed PR fix: resolve CI failures from dependency bump #172 while doing this refactor; its E2E fix is still preserved in the new runner design
• follow-up issue will track explicit capability-based coverage expansion

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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