AuthRequiredError and DynamicTransportError need constructors for external crates

[wpfleger96]

AuthRequiredError and DynamicTransportError were marked #[non_exhaustive] in #715 / #768, but neither has a constructor usable by external crates. Same situation as StoredCredentials (#777, fixed in #778).

AuthRequiredError has no constructor at all. External crates testing auth error handling paths (e.g. verifying that should_attempt_oauth_fallback correctly detects auth challenges) can't construct test fixtures without a serde roundtrip.

DynamicTransportError has ::new<T: Transport<R>>(), but it requires a real Transport impl as a type parameter. This makes it unusable for test fixtures that need a fake transport name and type ID.

We ran into both of these in block/goose after bumping to rmcp 1.4.0 (the same upgrade that surfaced the StoredCredentials issue).

Suggested fixes:

impl AuthRequiredError {
    pub fn new(www_authenticate_header: String) -> Self {
        Self { www_authenticate_header }
    }
}

impl DynamicTransportError {
    pub fn from_parts(
        transport_name: impl Into<Cow<'static, str>>,
        transport_type_id: std::any::TypeId,
        error: Box<dyn std::error::Error + Send + Sync>,
    ) -> Self {
        Self { transport_name: transport_name.into(), transport_type_id, error }
    }
}