improve docs around ssh key management for ascp plugin

lib/galaxy/config/sample/file_sources_conf.yml.sample

@@ -101,7 +101,7 @@
 #  port: 33001
 #  rate_limit: "500m"
 #  disable_encryption: false  # Enable encryption for security
-#  ssh_key: |
+#  ssh_key_content: |
 #    -----BEGIN RSA PRIVATE KEY-----
 #    <REPLACE WITH YOUR ACTUAL SSH PRIVATE KEY CONTENT>
 #    -----END RSA PRIVATE KEY-----

lib/galaxy/files/sources/ascp.py

@@ -83,18 +83,24 @@ class AscpFilesSource(FsspecFilesSource[AscpFilesSourceTemplateConfiguration, As
     """Galaxy file source plugin for Aspera ascp transfers.
 
     This plugin provides high-speed file downloads using the Aspera ascp protocol.
-    It requires the ascp binary to be installed and accessible, along with appropriate
-    SSH credentials configured.
+    It requires the ascp binary to be installed and accessible, along with SSH
+    private key content embedded in the configuration.
 
     Features:
     - Download-only functionality (no upload or browsing in base implementation)
     - Configurable transfer rate limiting
     - Optional encryption disabling for maximum speed
     - Support for both ascp:// and fasp:// URL schemes
+    - Automatic retry with exponential backoff for transient failures
+    - Resume support for interrupted transfers
+
+    Configuration:
+    - ssh_key_content: Required field containing the SSH private key as a string
+    - The key content is automatically written to a temporary file during transfers
+    - Temporary key files are securely cleaned up after each transfer
 
     The plugin is designed to be extensible for future enhancements such as:
     - Subclassing for ENA-specific handling
-    - Retry logic with exponential backoff
     - Fallback mechanisms to alternative protocols
     - Upload and browsing capabilities
     """

lib/galaxy/files/sources/ascp_fsspec.py

@@ -39,7 +39,7 @@ class AscpFileSystem(AbstractFileSystem):
 
     Args:
         ascp_path: Path to the ascp binary (default: "ascp")
-        ssh_key: SSH private key as string content OR path to key file
+        ssh_key: SSH private key content as a string (required)
         user: Username for ascp connection (e.g., "era-fasp")
         host: Hostname (e.g., "fasp.sra.ebi.ac.uk")
         rate_limit: Transfer rate limit (default: "300m")