op-challenger: never load the absolute prestate for permissioned games

[pauldowman]

Fixes #21470.

When a permissioned game (type 1) is configured with --prestates-url, op-challenger fetches the game's absolute prestate from that URL keyed by hash. Permissioned games are often configured with a placeholder prestate that isn't published there, so the fetch fails and op-challenger errors out creating the game player — even though it should still resolve the game. (With --cannon-prestate <file> this doesn't happen because the single-file source ignores the hash, hence the --cannon-prestate=any-string workaround.)

Permissioned games are played only by trusted actors, resolve at the output-root level, and never reach step(), so their VM prestate is never actually used. Per review feedback, the challenger now never loads the prestate for permissioned games at all (rather than only tolerating a failed fetch):

• The bottom prestate provider for the permissioned game type is an unconditional empty placeholder — no fetch, no cache, no error path. Cannon and other game types still hard-fail on a missing prestate.
• The prestate flags (--cannon-prestate / --prestates-url) are no longer required when only the permissioned game type is enabled, at both the CLI-flags and config validation layers, mirroring the existing op-program server relaxation. They are still required whenever the cannon game type is enabled.
• New GameType.IsPermissioned() and CannonFamilyGameTypes in game/types centralize the "permissioned ⇒ never reaches step()" property (already covering the upcoming SuperPermissionedGameType) and the cannon flag-family membership; the validators derive their own requirements from them instead of taking caller-supplied booleans.
• Tests pin: permissioned games get the placeholder without any load attempt; cannon still errors on a missing prestate; prestate flags are optional for permissioned-only configs but still required when cannon and permissioned are both enabled (config and CLI layers).
• README documents the behavior.

[Inphi]

what do you think about always returning the empty placeholder, rather than only when the prestate is unavailable.

[pauldowman]

That's a good idea. There's no reason to even attempt to fetch it if permissioned. Will do.