fix(cells): harden Ktor S3 transfers PR 2

[MohamadJaara]

Stack

• Depends on feat: add shared KMP S3 client for Cells transfers [WPB-26729] #4262.
• Review and merge feat: add shared KMP S3 client for Cells transfers [WPB-26729] #4262 before this PR.

Intent

Keep the shared KMP S3 migration review focused by moving defensive response handling and expanded edge-case coverage into a follow-up layer.

Changes

• Add strict namespace-aware parsing for S3 XML responses and embedded errors.
• Validate streamed download lengths without replaying partial destinations.
• Use a typed failure when Cells credentials are unavailable.
• Expand multipart, signing, retry, cancellation, and malformed-response coverage.
• Preserve the existing per-request token refresh, retry delay, and multipart cleanup behavior.

[github-actions]

Test Results

0 tests   - 4 962   0 ✅  - 4 942   0s ⏱️ - 8m 39s
0 suites  -    13   0 💤  -    20 
0 files    -    13   0 ❌ ±    0 

Results for commit 0d0bd18. ± Comparison against base commit 604f249.

♻️ This comment has been updated with latest results.