tests: add assertions and fix multipart test coverage

Add call count assertions to all non-compression S3 tests:
- multipart_success: assert CompleteMultipartUpload == 1
- putobject_success: assert PutObject == 1
- putobject_error: assert PutObject >= 1
- create_upload_error: assert CreateMultipartUpload >= 1
  and UploadPart == 0
- upload_part_error: assert UploadPart >= 1 and
  CompleteMultipartUpload == 0
- complete_upload_error: assert CompleteMultipartUpload >= 2

Add call count assertions to compression tests to verify
the correct upload path (PutObject or multipart) is used.

Use upload_timeout=6s with sleep(10) consistently across
all tests so the timer fires and exercises the real upload
path. Add unique store_dir via mkdtemp to multipart tests
to prevent cross-test contamination. Clean up all API call
counters at the end of each test.

Co-authored-by: Thean Lim <theanlim@amazon.com>
Signed-off-by: Anuj Singh <singholt@amazon.com>

Author: singholt