fix: keep transient Protun key in NetworkManager

[UglyEgg]

Problem

Protun supplies its WireGuard private key while constructing an unsaved NetworkManager profile, but marks the secret AGENT_OWNED. NetworkManager therefore asks a user-session secret agent to store and later return the key. Plasma has no Protun-specific NetworkManager secret plugin, so activation can remain blocked waiting for credentials.

NetworkManager defines flag 0 as system-owned and flag 1 as agent-owned: https://networkmanager.dev/docs/api/latest/secrets-flags.html

Change

Keep the supplied key on the existing transient NetworkManager profile and make save_to_disk=False explicit at the call site. The key, protocol, peer, routing, and tunnel configuration are unchanged.

An unsaved profile is not persistent configuration, but NetworkManager may represent in-memory profiles under root-only runtime storage. This proposal therefore removes the desktop secret-agent dependency; it does not claim the key exists only in process memory.

Verification

• Tests assert both the system-owned secret flag and explicit unsaved-profile contract.
• Complete upstream suite: 469 passed; two unchanged dependency warnings.
• Flake8 passes on both changed files.
• Downstream Fedora 44 / Plasma acceptance covered cold start, connect, disconnect, reconnect, and suspend/resume using the equivalent patch against Proton's packaged API Core 5.6.20.

Development disclosure: I developed this patch with material assistance from OpenAI Codex under my direction, then reviewed, tested, and signed the final change.