Update filebucket content acceptance tests for literal checksum semantics

[silug]

Short description

#170 removed the implicit filebucket retrieval for file content values that look like checksums; such content is now always managed literally. Two acceptance tests still exercised the removed behavior and fail on all platforms (acceptance run 31157133263, openvox suite shard:group3; also present but masked behind the lookup failures in run 31098859281):

• ticket_6541_invalid_filebucket_files.rb asserted that checksum-like content never overwrites the target and that a well-formed checksum retrieves bucket content. Rewritten as a regression test for the new semantics: checksum-like content -- malformed or well-formed -- is written literally, which is exactly the capability whose absence motivated the removal.

• resource/file/content_attribute.rb ended with a checksum-from-filebucket scenario; that block is removed with a comment pointing at the rewritten test. The raw-content, checksum-type, and illegal-timestamp coverage is unchanged.

Verified on a local beaker rig against openvox-agent 9.0.0~beta2: both tests reproduce the pipeline failures unmodified and pass with this change.

Generated by Claude Code

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)