StoredCredentials needs a constructor — #[non_exhaustive] without new() makes it unconstructable from external crates

[wpfleger96]

StoredCredentials was marked #[non_exhaustive] in #715 / #768, but unlike OAuthClientConfig (which got a new() in the same PR), StoredCredentials doesn't have a constructor or Default impl. External crates that implement CredentialStore and need to construct StoredCredentials in their save() path don't have a compile-safe way to do so.

The workaround we landed on is a serde JSON roundtrip:

let credentials: StoredCredentials = serde_json::from_value(serde_json::json!({
 "client_id": client_id,
 "token_response": token_response,
 "granted_scopes": granted_scopes,
 "token_received_at": timestamp,
}))?;

This bypasses #[non_exhaustive] via Deserialize but turns what should be a compile-time error (if a new required field is added) into a silent runtime failure during OAuth.

We ran into this in block/goose after bumping to rmcp 1.3.0 for Unix socket transport support (#749).

Suggested fix: A constructor matching the pattern used for OAuthClientConfig:

impl StoredCredentials {
 pub fn new(
 client_id: String,
 token_response: Option<OAuthTokenResponse>,
 granted_scopes: Vec<String>,
 token_received_at: Option<u64>,
 ) -> Self {
 Self {
 client_id,
 token_response,
 granted_scopes,
 token_received_at,
 }
 }
}