Refactor FilesExt: deduplicate complete-upload and simplify resumable upload headers

[parthban-db]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/refactor-files-client [Files changed]

---

What changes are proposed in this pull request?

This PR makes two small, behavior-preserving refactors to the FilesExt upload internals:

1. Deduplicate _complete_multipart_upload logic. The _perform_multipart_upload method had an inline copy of the complete-upload HTTP request that was identical to the existing _complete_multipart_upload method (already used by the parallel upload paths). The inline code is replaced with a call to the shared method. The shared method's body construction is also simplified from a loop to a list comprehension.
2. Simplify _abort_resumable_upload header handling. The resumable upload URL response returns headers as a list of {"name": ..., "value": ...} dicts. Previously, this list was converted to a flat dict in three separate places: once per upload-loop iteration and once inside _abort_resumable_upload. Now the conversion happens once after parsing the response, and the resulting dict is reused everywhere. The _abort_resumable_upload signature changes from required_headers: list to headers: dict[str, str], removing its internal conversion loop.

Why: These refactors reduce code duplication and make the upload internals easier to follow, preparing for a larger refactor that will introduce a _RequestBuilder abstraction to support both presigned-URL and storage-proxy upload paths.

How is this tested?

• Existing test with the following modification.
• Added two new assertions in the resumable upload test mock to verify that cloud-provider headers from the resumable URL response are correctly forwarded to both upload-part requests and abort requests. These assertions did not exist before, strengthening the test coverage for the header-passing contract that this refactor touches.

NO_CHANGELOG=true

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-py

Inputs:

• PR number: 1279
• Commit SHA: 03635fefee05aab5d366b8fe82e8160d249a7348

Checks will be approved automatically on success.