fix: isolate files in nested local operations

[rasitakyol]

What?

• Isolate req.file for child Local API create and update operations that reuse a caller request.
• Add create and update regression coverage for upload collections where a file is optional.

Why?

Nested Local API operations currently write their file state onto the shared request. A file-less child operation can therefore clear the outer request's file before later hooks use it.

How?

• Use the existing isolateObjectProperty utility so child file writes remain local while transaction, user, and context state stays shared.
• Verify that the caller retains the exact file reference and the child document remains file-less across adapters.
• Validated with Node 24.15.0 and pnpm 11.9.0: test/uploads/int.spec.ts passed 111/111 on SQLite and MongoDB, pnpm build:payload --force, Prettier, source ESLint, and git diff --check passed.

Fixes #15975

[rasitakyol]

CI note: the current failing job is E2E - lexical__collections__LexicalViewsProviderDefault [next]. Its two failures time out waiting for [data-lexical-editor="true"] in test/lexical/collections/LexicalViewsProviderDefault/e2e.spec.ts.

This PR only changes the local create/update upload handling and test/uploads/int.spec.ts; it does not change Lexical code. The full uploads integration suite passes locally with both SQLite (111/111) and MongoDB (111/111). All Green is failing only as the aggregate of that Lexical job.

Could a maintainer rerun the failed Actions job? I do not have permission to rerun workflows in the upstream repository.