Cleanup experimentalFeature/enablement/set

[mzeng-openai]

Why

experimentalFeature/enablement/set still allowed several keys that no longer need to be managed through this API. Keeping those keys also preserved corresponding special-case logic, including refreshing the apps list when the apps key was enabled.

The endpoint also rejected an entire request when any key was invalid or unsupported. That makes clients brittle when they send a mix of current and stale keys, even when the valid entries can still be applied safely.

What changed

• remove the feature keys that no longer need to be supported by experimentalFeature/enablement/set
• remove the corresponding apps-list refresh path and its auth/config plumbing
• ignore and warn on invalid or unsupported keys while still applying valid keys from the same request
• update the app-server documentation and integration coverage for the reduced key set and partial-acceptance behavior

Test plan

• just test -p codex-app-server experimental_feature_enablement_set (6 passed)
• just test -p codex-app-server exercised the changed tests successfully; unrelated sandbox-dependent and watcher/timing tests failed locally