GHSA-93xw-j965-9mx3: MCP Atlassian: Arbitrary file read/exfiltration via upload_attachment missing validate_safe_path()

## Summary The `upload_attachment` method in `confluence/attachments.py` reads and uploads arbitrary local files to Confluence without calling `validate_safe_path()`. Both download methods (`download_attachment` at line 223, `download_content_attachments` at line 272) correctly call `validate_safe_path()` before writing files, but the upload path at lines 35-79 skips this check entirely. An AI agent connected via MCP (or an attacker influencing that agent through prompt injection) can read any file on the host and exfiltrate it by uploading it as a Confluence page attachment. …