PROD-2307: adopt existing target containers by referenceName (mapping self-heal)

[5PK]

Summary

Defensive hardening for container-pusher — not the fix for PROD-2307's headline symptom (see caveats).

When the local mapping cache is absent (fresh machine, CI runner, wiped agility-files, or a create that succeeded server-side but was never mapped), container-pusher had no fallback: with no mapping row it always took the create path. Agility's saveContainer dedupes by reference name, so this rarely produced a visible duplicate container — but the missing mapping row is the real damage. Downstream content/pages resolve container references through hasValidMappings, and a missing container mapping tips content into the create path, which is what feeds duplicate accumulation across syncs.

Change

Mirrors the merged PROD-2211 map-on-adopt path in model-pusher: before creating, look for an existing target container matching by referenceName (unique per instance); if found, write the mapping row and skip instead of creating.

• Case-insensitive reference-name match
• Mapping write skipped under --preflight
• Same-name / different-model match logs a warning (adopts on the unique name)

What this does NOT do (scope)

• Does not resolve the ticket's divergent-hash nested-container doubling (orphans with names the source never had). That needs the source/target nested-container names to diverge, a path not reproducible on the current build — likely a ~13.16-era artifact. Cleanup of existing orphans is PROD-2282.
• The content-item duplication observed under a missing mapping cache is a distinct root cause, tracked separately.

Testing

• New container-pusher unit tests: adopt-by-referenceName, case-insensitive match, still-creates-when-no-match, preflight-no-write.
• Full suite green: 1732 tests / 98 suites.
• Live verification against test instances (d46a3d59-u → db707fc0-u): a fresh-state sync now reports containers adopted (skipped) with mapping rows written instead of re-created; a second sync is stable.

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