KNOWN_EXTRAS: frozenset[str] = MODEL_PROVIDER_EXTRAS | SANDBOX_EXTRAS | STANDALONE_EXTRASUnion of all individually-installable extras.
Excludes the composite meta-extras (all-providers, all-sandboxes) since
those expand to other extras and don't add anything on their own.
Drift-protected by test_model_config.TestProviderApiKeyEnv and the
model-provider-drift checks; new extras must be added to the corresponding
category frozenset above.