Function derive_with_policy 

pub async fn derive_with_policy(
    session: &Session,
    provider: Arc<dyn Provider>,
    model: &str,
    system_prompt: &str,
    tools: &[ToolDefinition],
    event_tx: Option<&Sender<SessionEvent>>,
    policy: DerivePolicy,
    force_keep_last: Option<usize>,
) -> Result<DerivedContext>

Derive an ephemeral DerivedContext under a chosen DerivePolicy.

Generalisation of derive_context that accepts a policy selector. DerivePolicy::Legacy delegates back to derive_context so the two signatures co-exist without behaviour drift during rollout.

Arguments

Same as derive_context, plus:

• policy — Which derivation strategy to run. See DerivePolicy.
• force_keep_last — When Some(n), bypass policy selection and fall back to the legacy keep-last derivation used for prompt-too-long recovery.

Errors

Propagates any error from the underlying pipeline.