feat(config): add centralized configuration constants and infrastructure tests

- Introduce config.rs with all magic numbers and resource limits defined as constants
- Add comprehensive test suite covering metrics rendering, rate limiting, and cache operations
- Include tests for configuration constant validation and sanitization functions
- Add pack protocol tests for index_pack and pack_objects functionality
- Implement remote repository discovery tests with security validations
- Support runtime overrides via environment variables for all configurable values

tests/pack_test.rs

@@ -0,0 +1,50 @@
+mod common;
+
+use gitks::pb::*;
+use tokio_stream::StreamExt;
+
+fn hdr() -> RepositoryHeader {
+    RepositoryHeader {
+        relative_path: "test-repo".into(),
+        ..Default::default()
+    }
+}
+
+#[test]
+fn test_index_pack_empty_data() {
+    let (_dir, gb) = common::setup_bare_repo();
+    let result = gb.index_pack(vec![IndexPackRequest {
+        repository: Some(hdr()),
+        data: vec![],
+        done: false,
+        strict: false,
+        keep: false,
+    }]);
+    assert!(result.is_err());
+}
+
+#[tokio::test]
+async fn test_pack_objects_all() {
+    let (_dir, gb) = common::setup_bare_repo();
+
+    let mut stream = gb
+        .pack_objects(PackObjectsRequest {
+            repository: Some(hdr()),
+            options: None,
+        })
+        .await
+        .unwrap();
+
+    let mut total_bytes = 0usize;
+    let mut has_error = false;
+    while let Some(chunk) = stream.next().await {
+        match chunk {
+            Ok(c) => total_bytes += c.data.len(),
+            Err(_) => has_error = true,
+        }
+    }
+    assert!(!has_error, "pack_objects stream had errors");
+    assert!(total_bytes > 0, "pack_objects produced no data");
+}
+
+