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/remote_test.rs

@@ -0,0 +1,56 @@
+use gitks::pb::*;
+use gitks::remote::find_remote::{find_remote_repository, find_remote_root_ref};
+
+#[test]
+fn test_find_remote_repository_empty_url() {
+    let resp = find_remote_repository(FindRemoteRepositoryRequest {
+        remote_url: String::new(),
+    })
+    .unwrap();
+    assert!(!resp.exists);
+    assert!(resp.refs.is_empty());
+}
+
+#[test]
+fn test_find_remote_repository_invalid_scheme() {
+    let resp = find_remote_repository(FindRemoteRepositoryRequest {
+        remote_url: "ftp://bad".into(),
+    });
+    assert!(resp.is_err());
+}
+
+#[test]
+fn test_find_remote_repository_file_scheme_blocked() {
+    let resp = find_remote_repository(FindRemoteRepositoryRequest {
+        remote_url: "file:///tmp/test.git".into(),
+    });
+    assert!(resp.is_err());
+}
+
+#[test]
+fn test_find_remote_root_ref_empty_url() {
+    let resp = find_remote_root_ref(FindRemoteRootRefRequest {
+        remote_url: String::new(),
+    })
+    .unwrap();
+    assert!(resp.ref_name.is_empty());
+    assert!(resp.target_oid.is_empty());
+}
+
+#[test]
+fn test_find_remote_root_ref_invalid_scheme() {
+    let resp = find_remote_root_ref(FindRemoteRootRefRequest {
+        remote_url: "ftp://bad".into(),
+    });
+    assert!(resp.is_err());
+}
+
+#[test]
+fn test_find_remote_root_ref_unreachable() {
+    // A valid scheme but unreachable host should return empty, not error
+    let resp = find_remote_root_ref(FindRemoteRootRefRequest {
+        remote_url: "https://0.0.0.0:1/nonexistent.git".into(),
+    })
+    .unwrap();
+    assert!(resp.ref_name.is_empty() || resp.target_oid.is_empty());
+}