refactor: deduplicate uploadClient test helpers and extract error code constant

[DaxServer]

• Export UPLOAD_CHUNK_FILE_EXCEPTION constant from client.ts (was an inline string literal duplicated across both error handlers and both tests)
• Extract mockImageFetch(), callUploadFile(), and makeChunkUploadClient() test helpers to eliminate repeated boilerplate across the uploadFile error paths describe block
• The two new StorageError tests from PR fix: map UploadChunkFileException to StorageError for retry #90 now use makeChunkUploadClient() and the shared constant instead of repeating 4-line setup each

— Claude Sonnet 4.6

[greptile-apps]

Confidence Score: 5/5

Safe to merge — this is a straightforward refactoring with no changes to production logic or test coverage.

The only production-code change is exporting a string constant that was already present verbatim in two places; both usages in client.ts are a direct substitution with no surrounding logic altered. The test changes mechanically extract repeated setup into helpers and reorder two StorageError tests (chunk → commit), which is strictly cosmetic. Each helper is narrow and correct: mockImageFetch isolates CDN fetch setup, callUploadFile fixes the argument list in one place, and makeChunkUploadClient bundles the three stubs every chunk-path test needs. No new behaviour is introduced and no existing assertions are dropped.

No files require special attention.

Important Files Changed

Filename	Overview
backend/src/mediawiki/client.ts	Extracts UPLOAD_CHUNK_FILE_EXCEPTION as an exported constant, replacing two identical inline string literals in the chunk-upload and commit error-handling branches — no logic change.
backend/src/tests/uploadClient.test.ts	Adds mockImageFetch(), callUploadFile(), and makeChunkUploadClient() helpers; refactors all uploadFile error paths tests to use them; reorders the two StorageError tests (chunk before commit); imports the new constant — behaviour unchanged.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Test: StorageError chunk] -->|makeChunkUploadClient| B[new MediaWikiClient]
    B --> C[mock getCsrfToken]
    C --> D[mock findDuplicates → empty]
    D --> E[mockImageFetch → 200 OK]
    E --> F[makeRedisMock]
    F --> G[return client, redis]
    G -->|override apiUploadChunk| H[mock: always returns error]
    H --> I[callUploadFile client, redis]
    I --> J{expects StorageError}

    A2[Test: StorageError commit] -->|makeChunkUploadClient| B
    G2[return client, redis] -->|override apiUploadChunk| K[mock: call1=Continue, call2=error]
    K --> I2[callUploadFile client, redis]
    I2 --> J2{expects StorageError}

    subgraph UPLOAD_CHUNK_FILE_EXCEPTION
        CONST["internal_api_error_MediaWiki\\Upload\\Exception\\UploadChunkFileException"]
        CONST -->|client.ts line 254| ERR1[chunk upload error handler]
        CONST -->|client.ts line 285| ERR2[commit error handler]
        CONST --> T1[chunk StorageError test]
        CONST --> T2[commit StorageError test]
    end

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