fix: ignore stray empty input in interactive prompts

When running interactive tests (like `INTERACTIVE=1 cargo test --nocapture`),
stray newlines from previous commands or input streams can cause `read_line`
to immediately return an empty string. This caused the first prompt
(`prompt_microstructure_variable`) to be skipped, and the intended
answer was instead consumed by the second prompt (`prompt_cutoff_value`).

This commit updates `helpers::prompt_microstructure_variable` and
`helpers::prompt_cutoff_value` to repeatedly read from `stdin` until a
non-empty line is provided. It also correctly handles EOF (`bytes == 0`)
to prevent infinite loops when stdin is exhausted.

Co-authored-by: soniapi <396009+soniapi@users.noreply.github.com>

Author: google-labs-jules[bot]

src/helpers.rs

@@ -11,9 +11,20 @@ pub fn prompt_microstructure_variable(connection: &mut PgConnection) -> Option<c
     let mut input = String::new();
 
     // Read user input
-    io::stdin()
-        .read_line(&mut input)
-        .expect("Failed to read input");
+    loop {
+        input.clear();
+        let bytes = io::stdin()
+            .read_line(&mut input)
+            .expect("Failed to read input");
+
+        if bytes == 0 {
+            return None; // EOF
+        }
+
+        if !input.trim().is_empty() {
+            break;
+        }
+    }
 
     // Check if input is "1)" or "1"
     let trimmed = input.trim();
@@ -52,9 +63,20 @@ pub fn prompt_cutoff_value() -> Option<f32> {
     let mut input = String::new();
 
     // Read user input
-    io::stdin()
-        .read_line(&mut input)
-        .expect("Failed to read input");
+    loop {
+        input.clear();
+        let bytes = io::stdin()
+            .read_line(&mut input)
+            .expect("Failed to read input");
+
+        if bytes == 0 {
+            return None; // EOF
+        }
+
+        if !input.trim().is_empty() {
+            break;
+        }
+    }
 
     // The user can only enter a number for now, anything else will exit the function.
     if let Ok(divide) = input.trim().parse::<f32>() {