fix(sites): deep-merge hlxConfig/deliveryConfig on PATCH to preserve content.source (SITES-49362)

[mimchome]

https://jira.corp.adobe.com/browse/SITES-49362

• Problem: updateSite replaced hlxConfig and deliveryConfig as whole objects. A PATCH sending hlxConfig with only rso+code (no content) silently erased hlxConfig.content.source, breaking crosswalk sites (deploy reports "Site setup incomplete").
• Fix: Deep-merge hlxConfig/deliveryConfig sub-keys into the existing value. Omitted sub-keys keep their existing value; a plain object merges recursively; arrays/scalars replace; an explicit null deletes a sub-key so callers can still remove one intentionally. Omitting the field entirely leaves it unchanged (unchanged behavior).
• Safe by default: every caller now preserves omitted sub-keys automatically — no client change needed to stop the data loss.
• authoringType handling and the hlxConfigChanged/deliveryConfigChanged deepEqual checks are unchanged (they now compare against the merged result).
• Tests: added unit tests in test/controllers/sites.test.js — partial hlxConfig preserves existing content.source; hlxConfig.content: null deletes it; omitting hlxConfig leaves it unchanged; plus the same partial-merge case for deliveryConfig. Full sites controller suite passes (341) and lint is clean.

[github-actions]

This PR will trigger a patch release when merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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