Object keys are not URL-encoded in multipart upload methods

[Nana-EC]

Description

uploadTextFile() and downloadTextFile() correctly call urlEncode(key, true) when
building their request URLs. However, createMultipartUpload(), multipartUploadPart(),
completeMultipartUpload(), and abortMultipartUpload() concatenate the object key directly
into the URL without encoding. Object keys containing spaces, +, #, or other reserved
characters will produce malformed URLs, incorrect SigV4 signatures, and failed requests.

Steps to reproduce

1. Call uploadFile() with an objectKey containing a space (e.g., "my folder/file.txt").
2. The internally called createMultipartUpload() constructs:
   endpoint + bucketName + "/" + key + "?uploads="
   The space is not encoded, producing a broken URL.

Expected behavior
All methods that embed an object key in a URL should call urlEncode(key, true).

Actual behavior
Keys with special characters produce malformed URLs and authentication failures.

Affected file
client/src/main/java/com/hedera/bucky/S3Client.java — lines ~436, ~481, ~530, ~400

Additional context

No response

Hedera network

No response

Version

v0.1.0

Operating system

None