Pass --fixed-fps as two argv tokens

[Eliasepfl]

I have been building a personal project on top of godot-rl-agents, a harness that generates Godot games from text and trains a policy on each one to check it is actually solvable. Getting it to run at scale surfaced a handful of small bugs and rough edges in the library. This is the first of four PRs, all independent and small, mostly fixes and throughput things I needed. Happy to rework or drop any of them.

--fixed-fps is a Godot engine argument and expects its value as a separate token, but it is appended as one string:

launch_cmd.append(f"--fixed-fps {framerate}")

Popen receives argv as a list, so Godot gets a single unknown parameter --fixed-fps 60 rather than the flag and its value. On 4.7 the engine then gives up before starting the game:

$ Godot_v4.7-stable_linux.x86_64 --headless --path project --quit "--fixed-fps 60"
ERROR: Couldn't detect whether to run the editor, the project manager or a specific project. Aborting.

$ Godot_v4.7-stable_linux.x86_64 --headless --path project --quit --fixed-fps 60
(gets past argument parsing and runs the project)

So passing any framerate to GodotEnv currently stops the game from launching.

The fix splits it into two tokens. I also pulled the argv construction into GodotEnv._build_launch_cmd so it can be unit tested without a game binary, and added tests for the flag forms. Everything else keeps the --key=value shape the plugin parses.