Calculate the forward extremities in Rust

Port `_calculate_new_extremities`, `_calculate_new_state_dag_extremities`
and `_prune_extremities` from the persist events controller to
`rust/src/persist/extremities.rs`. They read through an `ExtremityReads`
trait, which the `PersistReader` implements and the unit tests implement
over an in-memory DAG. Metrics go to a `PersistCounts` the caller owns,
so shadow mode can drop them.

Missing prev state events fail with `MissingPrevStateEvents`, which
`persist_error_to_py` turns into the same 500 `SynapseError` as Python.

`PersistReaderForTest` gains the three calculations, and
`test_persist_reader.py` compares each with the Python method on the
same database and the same new events.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UHKYPSc37kWqGACn9aKxKL

rust/src/persist/python.rs

@@ -18,10 +18,14 @@
 
 use std::sync::Arc;
 
+use http::StatusCode;
 use pyo3::{exceptions::PyRuntimeError, intern, prelude::*, types::PyDict};
 
-use super::{context::PersistContext, metrics::PersistMetrics};
+use super::{
+    context::PersistContext, extremities::MissingPrevStateEvents, metrics::PersistMetrics,
+};
 use crate::{
+    errors::SynapseError,
     events::Event,
     homeserver::HomeServer,
     runtime::RustRuntime,
@@ -122,6 +126,28 @@ impl Persister {
     }
 }
 
+/// Convert an error from persisting to the Python exception the Python
+/// path would raise: a Python exception as it was raised, a 500
+/// `SynapseError` for missing prev state events, as
+/// `_calculate_new_state_dag_extremities` raises, and a `RuntimeError`
+/// otherwise.
+pub fn persist_error_to_py(e: anyhow::Error) -> PyErr {
+    let e = match e.downcast::<PyErr>() {
+        Ok(err) => return err,
+        Err(e) => e,
+    };
+    if let Some(missing) = e.downcast_ref::<MissingPrevStateEvents>() {
+        return SynapseError::new(
+            StatusCode::INTERNAL_SERVER_ERROR,
+            missing.to_string(),
+            "M_UNKNOWN",
+            None,
+            None,
+        );
+    }
+    PyRuntimeError::new_err(format!("{e:#}"))
+}
+
 /// `(internal_metadata, json)`: the two `event_json` columns as Rust would
 /// write them for `event`, for tests to compare with Python's.
 #[pyfunction]