Add GIL-releasing block_on helpers on the shared tokio runtime

Add the `block_on`/`block_on_result`/`block_on_next` helpers the Postgres
backend uses to drive its async `tokio-postgres` futures to completion from
sync, GIL-holding Python methods, releasing the GIL for the wait. They take
a `tokio::runtime::Handle` and block on it from the calling (Python) thread.

Rather than give the DB backend a runtime of its own, they use the
extension's existing shared runtime (`tokio_runtime::PyTokioRuntime`, stored
on the reactor). `start` is made idempotent and a `runtime_handle` accessor
starts it on demand, so a caller that needs a connection before the reactor
is running still gets a handle; once the reactor runs, its
`callWhenRunning(start)` hook is a no-op.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RPEeXx2fAG67o6u4CnmC8W

rust/src/database/postgres/mod.rs

@@ -7,14 +7,16 @@
 //! The driver itself is async; the eventual `Connection` / `Cursor` types will
 //! drive it from sync Python methods via a shared multi-thread tokio runtime.
 
+use pyo3::exceptions::PyRuntimeError;
 use pyo3::prelude::*;
 use pyo3::types::PyModule;
 
-// `pub` (rather than private) so the value-mapping types are reachable from the
-// crate root as public API while nothing inside the crate consumes them yet.
-// This is what stops clippy's `dead_code` lint from firing on them before the
+// `pub` (rather than private) so the not-yet-consumed public items in these
+// submodules are reachable from the crate root as public API. This is what
+// stops clippy's `dead_code` lint from firing on them before the
 // cursor/connection code (added in later changes) wires them up; the visibility
 // is tightened back to private once that happens.
+pub mod helpers;
 pub mod value;
 
 /// Register the `postgres` submodule under the parent `database` module.
@@ -35,3 +37,8 @@ pub fn register_module(py: Python<'_>, m: &Bound<'_, PyModule>) -> PyResult<()>
 
     Ok(())
 }
+
+/// Map a [`tokio_postgres`] error into a Python `RuntimeError`.
+fn pg_err_to_py(e: tokio_postgres::Error) -> PyErr {
+    PyRuntimeError::new_err(format!("postgres error: {e}"))
+}