fix(ui): prevent autosave from reverting recently edited fields

[lucasvass1]

Fixes payloadcms#17676

Summary

• Autosave could overwrite recently typed field values: switching to a different field while a previous autosave request was still in flight cleared that field's "modified" protection, so the stale response (with an older/partial value) could land and overwrite the newer edit. Fixed by tracking a per-field modifiedAt timestamp and comparing it against when each request was sent — a response is only accepted for a field if no local edit happened to it after that specific request was dispatched.
• Media library (and any relationship/upload) list drawer would flicker/reload during editing: unmodified object/array-valued fields (relationship, upload) got a brand new, content-identical value reference on every accepted autosave merge, which cascaded into useListDrawer recomputing and remounting the open drawer. Fixed by preserving the existing reference in mergeServerFormState when the incoming value is deeply equal to the current one.

Test plan

• Added an e2e regression test reproducing the reported race (type in one field, switch to another before autosave completes) — fails without the fix, passes with it
• Added an e2e test asserting an open relationship list drawer isn't remounted by an unrelated autosave — fails without the fix, passes with it
• Added a unit test in mergeServerFormState covering reference preservation for content-identical values
• Existing autosave/computed-field tests in test/form-state still pass (no regression to the "don't overwrite a field being actively edited" behavior)
• pnpm run build:core and lint pass

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[lucasvass1]

Opened this by mistake against this reproduction repo instead of the actual payloadcms/payload repository. Closing this and moving the fix to the correct PR:

payloadcms#17697