fix(core): preserve full entries returned by preSave handlers

[sridharkalaibala]

Summary

Fixes #7966. A preSave handler returning entry.setIn(['i18n', locale, 'data', 'title'], value) is currently wrapped inside the entry's data. This causes internal entry metadata to be saved as default-locale content and leaves the translation unchanged.

Preserve complete entry returns directly, while continuing to wrap field-data returns. Recognize the full entry by its Immutable Map shape and the data, collection, slug, and path keys supplied by createEntry; checking only data would misclassify a legitimate content field with that name. The event still returns the complete entry, preserving the file-collection fix from #7667.

Test plan

• Four new tests cover full-entry returns, mixed full-entry/data-only handler chains (including an undefined return), a content field named data, and passing a translated entry through the actual multiple-folder i18n file generator. The output assertions verify intact English content and updated Portuguese content in separate files.
• The first two regression tests failed against the original registry implementation before the fix. Focused registry, backend, and i18n suites: 142 tests pass, including the existing file-collection slug-preservation test.
• Full pnpm test passes: lint (JavaScript, CSS, formatting), TypeScript, 4 Cypress-runner tests, and 1,388 Jest tests across 112 suites with 138 snapshots. The existing 1,304 skipped tests / 2 skipped suites remain skipped.
• git diff --check passes. No live GitHub/GitLab save or browser E2E was run. No dependency or lockfile changes.
• AI assistance: developed and validated with OpenAI Codex.

Checklist

• I have read the contribution guidelines.

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