Shard state unit tests under Bazel (#37478)

## Why

Windows runs the state crate's database-heavy Rust tests serially within each
shard.

## What changed

Configure `state-unit-tests` to run across four Bazel shards.

GitOrigin-RevId: f540f3c157734a2b9944bae9af554ad9dbc4a69b

codex-rs/state/BUILD.bazel

@@ -11,4 +11,8 @@ codex_rust_crate(
         "thread_history_migrations/**",
     ]),
     crate_name = "codex_state",
+    test_shard_counts = {
+        # Windows runs database-heavy Rust tests serially within each shard.
+        "state-unit-tests": 4,
+    },
 )