Unified configuration semantics

Coding standard: Centralize configuration/credential semantics and never “re-derive” them in multiple places. Read from the real sources (env/config and provider-specific runtime credential caches), preserve provider identity during legacy interop, and let canonical normalizers own final resolution.

Apply this as follows:

• Single source of truth: Put “is this provider usable/configured?” logic in one shared function/module used by both server routes and the CLI picker.
• Use real credential signals: If credentials live outside declarative metadata (e.g., OAuth token caches), the usable gate must consult the provider’s inventory/registration signal rather than only ProviderMetadata config keys.
• Correct precedence & explicitness: Treat defaults differently from credentials. Don’t mark a provider usable just because a defaulted required key exists—require explicit setting and/or a real secret value.
• Don’t short-circuit normalization: Endpoints should avoid pre-filling fields that cause normalize_* logic to skip important normalization steps; pass raw model inputs to the canonical normalizer.
• Legacy migration correctness: When translating legacy flat keys to structured provider state, ensure updates apply to the target provider (not the currently active provider), and consider env overrides so persistence doesn’t accidentally attach to the wrong provider.

Example pattern (shared usable gate):

// Single source of truth used by both server + CLI
pub fn provider_is_usable(meta: &ProviderMetadata, provider_type: ProviderType) -> bool {
    // handle local override
    if meta.name == "local" { return true; }

    // OAuth/token-cache-backed providers must consult inventory/configured signals
    // (not just meta.config_keys)
    if meta.config_keys.iter().any(|k| k.oauth_flow) {
        return provider_inventory_configured(meta.name.as_str());
    }

    // For non-OAuth providers: require explicit config/secret presence (not only defaults)
    required_keys_all_present_and_secrets_real(meta)
}