fix(lib-storage): respect user-provided partSize option for Upload

[smilkuri]

Issue

#7379

Description

Fixes a bug where the partSize option in the Upload class constructor was completely ignored, causing all multipart uploads to use the calculated part size (max of 5MB and contentLength/10000) regardless of user preference.

Testing

Locally

dev-dsk-smilkuri-1a-17aece6b % yarn test

 RUN  v3.2.4 /local/home/smilkuri/aws-sdk-js-v3/lib/lib-storage

 ✓ src/chunks/getChunkUint8Array.spec.ts (6 tests) 6ms
 ✓ src/chunks/getDataReadableStream.spec.ts (4 tests) 165ms
 ✓ src/chunks/getDataReadable.spec.ts (3 tests) 165ms
 ✓ src/index.spec.ts (1 test) 2ms
 ✓ src/Upload.spec.ts (35 tests) 23661ms
   ✓ Upload > should add tags to the object if tags have been added multi-part  23588ms

 Test Files  5 passed (5)
      Tests  49 passed (49)
   Start at  21:53:02
   Duration  24.66s (transform 849ms, setup 0ms, collect 2.15s, tests 24.00s, environment 1ms, prepare 584ms)

Checklist

• [n/a] If the PR is a feature, add integration tests (*.integ.spec.ts).
• [n/a] If you wrote E2E tests, are they resilient to concurrent I/O?
• [n/a] If adding new public functions, did you add the @public tag and enable doc generation on the package?

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