fix(storage): prevent silent data loss in VikingFS.append_file()

[RGB-loop]

Summary

VikingFS.append_file() catches all exceptions when reading existing file content and silently falls back to an empty string. This means any transient AGFS failure (timeout, connection error, decryption failure) causes the subsequent write to overwrite the entire file with only the new content — silently losing all previous data.

The only caller is Session._append_to_jsonl(), which appends every message to messages.jsonl. A single read failure during append wipes the entire message history.

Changes

• Only suppress AGFSHTTPError with status 404 (file not found), which is the expected case on first write
• Let other AGFS errors (AGFSClientError: connection refused, timeout, etc.) propagate to the outer handler which logs and raises IOError

Trigger conditions

• AGFS backend temporary I/O error (network flap, S3 timeout)
• Encryption key rotation causing _decrypt_content to fail
• File encoding corruption causing _decode_bytes to raise

Test plan

• Verify first-time append still works (file does not exist → 404 → creates new file)
• Verify append to existing file works (read succeeds → content concatenated)
• Verify AGFS timeout during read raises IOError instead of silently overwriting

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Failed to generate code suggestions for PR