bazel: use native rust test sharding

Configure the large Rust test targets with Bazel's native shard_count while keeping rules_rust's stable test-name hash bucket assignment.

This depends on hermeticbuild/rules_rs#94, which pins the rules_rust stable sharding stack, and keeps the original test labels so BuildBuddy can render Bazel's native sharded test UI.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/BUILD.bazel

@@ -4,5 +4,9 @@ codex_rust_crate(
     name = "app-server",
     crate_name = "codex_app_server",
     integration_test_timeout = "long",
+    test_shard_counts = {
+        "app-server-all-test": 8,
+        "app-server-unit-tests": 8,
+    },
     test_tags = ["no-sandbox"],
 )