fix: add site and LLMO access validation in LlmoController. VULN-33517

[calvarezg]

Changes

• getLlmoConfig endpoint: Added getSiteAndValidateLlmo validation at the start of the try block (line 398). Ensures:
  -- Site exists
  -- LLMO is enabled for the site
  -- User has LLMO entitlement for the organization
• updateLlmoConfig endpoint (line 424): Added getSiteAndValidateLlmo validation at the start of the try block (line 437). -- Same validations as above.

Both endpoints now validate token and access before reading or updating LLMO config. Errors from getSiteAndValidateLlmo are handled by existing catch blocks and returned as badRequest responses, consistent with other endpoints.

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Related Issues

https://jira.corp.adobe.com/browse/VULN-33517

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✅ All modified and coverable lines are covered by tests.

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