fix: address PR review — rejection sampling and robust test

- Use rejection sampling to eliminate modulo bias in generate_code().
  Values above the largest multiple of 1_000_000 in u32 are discarded
  and re-drawn (~0.02% rejection rate).
- Make generate_code_is_not_deterministic test robust against the
  1-in-10^6 collision chance by trying 10 pairs instead of one.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/security/pairing.rs

@@ -150,10 +150,23 @@ fn generate_code() -> String {
     // UUID v4 uses getrandom (backed by /dev/urandom on Linux, BCryptGenRandom
     // on Windows) — a CSPRNG. We extract 4 bytes from it for a uniform random
     // number in [0, 1_000_000).
+    //
+    // Rejection sampling eliminates modulo bias: values above the largest
+    // multiple of 1_000_000 that fits in u32 are discarded and re-drawn.
+    // The rejection probability is ~0.02%, so this loop almost always exits
+    // on the first iteration.
+    const UPPER_BOUND: u32 = 1_000_000;
+    const REJECT_THRESHOLD: u32 = (u32::MAX / UPPER_BOUND) * UPPER_BOUND;
+
+    loop {
         let uuid = uuid::Uuid::new_v4();
         let bytes = uuid.as_bytes();
         let raw = u32::from_le_bytes([bytes[0], bytes[1], bytes[2], bytes[3]]);
-    format!("{:06}", raw % 1_000_000)
+
+        if raw < REJECT_THRESHOLD {
+            return format!("{:06}", raw % UPPER_BOUND);
+        }
+    }
 }
 
 /// Generate a cryptographically-adequate bearer token (hex-encoded).
@@ -314,11 +327,15 @@ mod tests {
 
     #[test]
     fn generate_code_is_not_deterministic() {
-        // Two codes generated in the same process should differ (with overwhelming
-        // probability — collision chance is 1 in 1,000,000).
-        let c1 = generate_code();
-        let c2 = generate_code();
-        assert_ne!(c1, c2, "Two consecutive codes should differ (CSPRNG)");
+        // Two codes should differ with overwhelming probability. We try
+        // multiple pairs so a single 1-in-10^6 collision doesn't cause
+        // a flaky CI failure. All 10 pairs colliding is ~1-in-10^60.
+        for _ in 0..10 {
+            if generate_code() != generate_code() {
+                return; // Pass: found a non-matching pair.
+            }
+        }
+        panic!("Generated 10 pairs of codes and all were collisions — CSPRNG failure");
     }
 
     #[test]