feat: report caller org ASO tier on cross-org sites-resolve

[sandsinh]

When callerImsOrg is present on GET /sites/resolve and differs from the target org's imsOrgId (a cross-org caller — e.g. an AEC shell in org A viewing org B), the asoTier field now reports the caller org's ASO tier instead of the target org's, so the UI gates on the shell's own tier.

• Applies to every asoTier emission (success and failure) for the resolved target org.
• isSummitPlgEnabled stays target-derived — only asoTier is overridden.
• Falls back to the target org's tier when the caller org is not in the DB (its tier is unknowable); no override when callerImsOrg matches the target.
• Org-not-found / access-denied paths keep asoTier: null unchanged.

Adds unit tests and updates the OpenAPI callerImsOrg/asoTier docs.

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Related Issues

https://jira.corp.adobe.com/browse/SITES-48838

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This PR will trigger a minor release when merged.