[Bug] - REST CopyObject, POSIX backend - attempt to add directory object w/same basename as non-dir obj results in confusing error.

[lrm25]

Describe the bug
The error is:

<?xml version="1.0" encoding="UTF-8"?>
   <Error><Code>InvalidRequest</Code><Message>This copy request is illegal because it is trying to copy an object to itself without changing the object&#39;s metadata, storage class, website redirect location or encryption attributes.</Message><Resource></Resource><RequestId></RequestId><HostId></HostId></Error>

The error makes some sense, since trying to add a folder to another folder in POSIX, when a file with the same name already exists in that folder, results in a "file exists" error. However, this operation is legal in direct-to-s3, and may be confusing to someone coming from s3 who doesn't understand why this doesn't work with a POSIX backend. One suggestion is be similar to the s3api error response: "Directory object contains data payload", if the object contains data, or something similar to: "a directory object cannot have the same name as a non-directory object in POSIX mode."

To Reproduce

1. Start versity with a posix backend, e.g.: ./versitygw --access=******** --secret=******** --region=us-east-1 --iam-dir=/Users/lukemccrone/devel/versitygw/iam --cert /Users/lukemccrone/devel/versitygw/cert.pem --key /Users/lukemccrone/devel/versitygw/versitygw.pem posix --versioning-dir /tmp/versioning /tmp/gw
2. Create bucket, e.g.: aws --no-verify-ssl s3api create-bucket --bucket versity-gwtest-bucket-one-1
3. Create, and add an object to the bucket, e.g.: aws --no-verify-ssl s3api put-object --body /Users/lukemccrone/devel/versitygw/versity-gwtest-files/a --bucket versity-gwtest-bucket-one-1 --key a
4. Use to script at https://github.com/versity/versitygw/blob/test/directory_objects/tests/rest_scripts/copy_object.sh to send a command to copy 'a' to 'a/'. The curl command should look like this: curl -ks -w %{http_code} -X PUT https://localhost:7070/versity-gwtest-bucket-one-1/a/ -H Authorization: AWS4-HMAC-SHA256 Credential=********/20250109/us-east-1/s3/aws4_request,SignedHeaders=host;x-amz-content-sha256;x-amz-copy-source;x-amz-date,Signature=56ab338dbace53054d43c22a67837c861f9d3daa379dc71a2cc5b221570d0641 -H x-amz-content-sha256: UNSIGNED-PAYLOAD -H x-amz-copy-source: versity-gwtest-bucket-one-1/a -H x-amz-date: 20250109T215702Z -o /Users/lukemccrone/devel/versitygw/versity-gwtest-files/result.txt

Expected behavior
A clearer error when the copy fails.

Server Version
Version : v1.0.9
Build : 684d053
BuildTime: 2025-01-09_11:04:12PM
Darwin Lukes-MacBook-Pro.local 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:25 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6030 arm64