Fix duplicate gateway upload filenames

[ZenCGL]

Motivation

• Multipart uploads to the Gateway could silently overwrite earlier parts when multiple form fields used the same basename, losing data for the overwritten file.

Description

• Use the shared claim_unique_filename helper to auto-rename duplicate filenames within a single POST /api/threads/{thread_id}/uploads request by adding _N suffixes and track them with a seen_filenames set so later parts do not truncate earlier ones in the same request (app/gateway/routers/uploads.py).
• Include original_filename in the upload response when a part was renamed so callers/agents can see the original basename (app/gateway/routers/uploads.py).
• Add a regression test test_upload_files_auto_renames_duplicate_form_filenames proving two form parts named data.txt are persisted as data.txt and data_1.txt with preserved contents (tests/test_uploads_router.py).
• Update backend docs to mention the new auto-rename behavior in uploads (backend/README.md, backend/CLAUDE.md).

Testing

• Ran the uploads router unit tests: cd backend && PYTHONPATH=. uv run pytest tests/test_uploads_router.py -q, all tests passed (29 passed).
• Ran the linter/formatter check: cd backend && uv run ruff check app/gateway/routers/uploads.py tests/test_uploads_router.py and fixed import ordering; final check passed.

Fixes #2795 -

[CLAassistant]

All committers have signed the CLA.

[WillemJiang]

@ZenCGL, thanks for your contribution. Please create a GitHub issue for the bug you found.
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[Copilot]

Pull request overview

Prevents silent overwrites when a single multipart upload request contains multiple parts with the same filename by ensuring per-request filename uniqueness and exposing the original name when auto-renaming occurs.

Changes:

• Deduplicate filenames within POST /api/threads/{thread_id}/uploads using claim_unique_filename(...) and track seen names per request.
• Include original_filename in the upload response when a file is renamed due to duplication.
• Add a regression test and update backend docs to describe the new auto-rename behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
backend/app/gateway/routers/uploads.py	Tracks per-request seen filenames and auto-renames duplicates; adds original_filename in response when renamed.
backend/tests/test_uploads_router.py	Adds regression test ensuring duplicate form filenames persist as distinct files with preserved contents.
backend/README.md	Documents auto-renaming behavior for duplicate filenames in a single upload request.
backend/CLAUDE.md	Notes the duplicate-filename auto-rename behavior in the file upload feature description.