test: deepen and complete project-wide test coverage (#297)

* test: deepen coverage for health doctor provider and tunnels

* test: add broad trait and module re-export coverage

src/agent/mod.rs

@@ -1,3 +1,16 @@
 pub mod loop_;
 
 pub use loop_::run;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn assert_reexport_exists<F>(_value: F) {}
+
+    #[test]
+    fn run_function_is_reexported() {
+        assert_reexport_exists(run);
+        assert_reexport_exists(loop_::run);
+    }
+}