refactor: narrow exclusive write lock duration in App.add_exception_handler #288

Problem Description

In src/lib.rs#L738-L755:

    fn add_exception_handler(
        &self,
        exc_type: pyo3::Bound<'_, pyo3::types::PyType>,
        handler: Py<PyAny>,
    ) -> PyResult<()> {
        let mut st = self.state.write();
        let is_async = pyo3::Python::with_gil(|py| -> PyResult<bool> {
            let inspect = py.import(inspect)?;
            inspect
                .getattr(iscoroutinefunction)?
                .call1((&handler,))?
                .extract::<bool>()
        })
        .unwrap_or(false);
        Arc::make_mut(&mut st.exception_handlers).push((exc_type.unbind(), handler, is_async));
        st.rebuild_snapshot();
        Ok(())
    }

The exclusive write lock self.state.write() is acquired before dynamically importing inspect and invoking inspect.iscoroutinefunction. This unnecessarily extends the exclusive write lock duration across multiple Python attribute lookups and function calls.

Proposed Solution

1. Receive py: Python<'_> in the method signature or compute is_async before taking self.state.write().
2. Restrict state.write() only to the push and st.rebuild_snapshot() operations.

Acceptance Criteria

• iscoroutinefunction check is performed prior to acquiring the write lock.
• Exclusive write lock duration is minimized.