fix: preserve req.file across nested Local API calls from hooks

[vjymisal0]

What?

req.file was being dropped when a hook called the Local API (e.g. payload.create/payload.update) and passed the original req through, per the documented pattern for preserving request context in nested hook calls.

Why?

createLocalReq mutates and returns the same req object passed via options.req — it does not clone it. In both createLocal (packages/payload/src/collections/operations/local/create.ts) and updateLocal (packages/payload/src/collections/operations/local/update.ts), the code unconditionally ran:

req.file = file ?? (await getFileByPath(filePath!))

When neither file nor filePath is passed to the nested Local API call (the normal case when a hook just forwards req), getFileByPath(undefined) resolves to undefined, overwriting whatever req.file was already set to by the in-flight upload operation. Any later hook (or a storage adapter like @payloadcms/storage-azure) that reads req.file off the same req then silently sees undefined.

How?

Only assign req.file when a file or filePath was explicitly provided to the Local API operation:

if (file || filePath) {
  req.file = file ?? (await getFileByPath(filePath!))
}

This preserves an existing req.file reference when the nested call isn't itself an upload, while leaving upload behavior for file/filePath options unchanged.

Added a regression test in test/uploads/int.spec.ts (req.file preservation across nested Local API calls) that sets req.file, calls payload.create on a non-upload collection with that req passed through (mirroring the hook pattern from the issue), and asserts req.file is still defined and unchanged afterward.

Test status

I was unable to run the integration test suite in my current sandbox due to environment constraints (insufficient local disk space to complete pnpm install for the full monorepo, and a Node version mismatch against the >=24.15.0 requirement in CLAUDE.md). I traced the fix and test logic through the code manually and am confident in the change, but flagging this explicitly rather than claiming a green run I couldn't produce. Happy to iterate on CI feedback.

Fixes #15975