docs: compile ES→OpenSearch migration tester guide

Summary

Compile the existing ES → OpenSearch migration documentation into a single, tester-facing getting-started guide, and bring the two source docs it draws from into agreement with the current state of the migration.

The migration is close to end-to-end complete (Phase 3 included) and moving into broad QA, but the reference material was scattered across three internal docs (architecture, client configuration, test plan) written for engineers. This adds one entry point a customer/tester can follow from the outside — bring up the lab, understand the phases, exercise search and index endpoints — without reading source.

What was done

New: docs/backend/OPENSEARCH_MIGRATION_TESTER_GUIDE.md — a compilation/onboarding guide that distills the three existing docs into an observable, tester-oriented walkthrough:

• Objective and the four migration phases, in observable terms.
• Quickstart on the self-contained single-node-os-migration Docker stack (with the compose permalink as a starting point), services/ports/credentials, and how to get the files without a full checkout.
• The configuration a tester actually touches, including the .os index-visibility role (OS_MIGRATION_INDEX_VISIBILITY_ROLE_KEY).
• Verification helpers (list/count indices, indicies table, reading the phase, recognizing the automatic migration shutdown).
• What to validate per phase, VTL viewTool regressions ($estool / $sitesearch, aggregations, raw-Contentlet field shadowing), and the search / index-access REST endpoints.
• Operational notes (Site Search crawl behavior, rollback/drift, restart-required on phase setup) and troubleshooting.

Updated for consistency with current behavior:

• docs/backend/OPENSEARCH_MIGRATION.md — added a "restart required" note to Phase Transitions: the flag is live only for routing, but the one-time phase setup (validation, automatic shutdown, OS index bootstrap) runs only at startup.
• docs/backend/OPENSEARCH_MIGRATION_TEST_PLAN.md — updated the testable scope (all phases, including Phase 3, are now testable end-to-end), the same restart note, and corrected the verbatim automatic-shutdown log lines to match the code.

All claims were cross-checked against the codebase.

Scope / notes

• Documentation only — no code changes. QA not needed.