vext-email-gateway: migrate OsRng to rand 0.9 try_fill_bytes API

Closes the pre-existing rand_core 0.9 build break flagged in commit
0d11476's report (the .expect("TODO") sweep). 3 sites used the rand
0.8 fallible-via-RngCore pattern:

  use rand::RngCore;
  let mut rng = rand::rngs::OsRng;
  rng.fill_bytes(&mut sk);

In rand 0.9 OsRng no longer implements `RngCore`; it implements
`TryRngCore` instead (the fallible variant) since OS-level RNG calls
can fail. Migrated to:

  use rand::TryRngCore;
  rng.try_fill_bytes(&mut sk)
      .expect("OsRng must succeed for cryptographic key generation");

Sites:
  * src/types.rs::tests::generate_signing_key (test scaffold)
  * src/types.rs::verification::generate_signing_key (#[cfg(feature = "kani")])
  * src/main.rs::get_or_create_identity (production; also marked
    INSECURE - prototype only in the surrounding comment, so .expect
    matches the existing risk acceptance)

Verified locally:
  cargo build  -> EXIT 0 (warnings unchanged)
  cargo test   -> 7 passed / 0 failed / 0 ignored

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

vext-email-gateway/src/main.rs

@@ -298,13 +298,14 @@ impl Gateway {
 
     /// Get or create cryptographic identity for email address
     fn get_or_create_identity(&self, _email: &str) -> Result<ed25519_dalek::SigningKey> {
-        use rand::RngCore;
+        use rand::TryRngCore;
 
         // TODO: Persistent storage of email → keypair mapping
         // For now, generate a new signing key (INSECURE - prototype only).
         let mut rng = rand::rngs::OsRng;
         let mut sk = [0u8; 32];
-        rng.fill_bytes(&mut sk);
+        rng.try_fill_bytes(&mut sk)
+            .expect("OsRng must succeed for cryptographic key generation");
         Ok(ed25519_dalek::SigningKey::from_bytes(&sk))
     }
 }

vext-email-gateway/src/types.rs

@@ -331,10 +331,11 @@ mod tests {
     use proptest::prelude::*;
 
     fn generate_signing_key() -> SigningKey {
-        use rand::RngCore;
+        use rand::TryRngCore;
         let mut rng = rand::rngs::OsRng;
         let mut sk = [0u8; 32];
-        rng.fill_bytes(&mut sk);
+        rng.try_fill_bytes(&mut sk)
+            .expect("OsRng must succeed for cryptographic key generation");
         SigningKey::from_bytes(&sk)
     }
 
@@ -401,10 +402,11 @@ mod verification {
     use super::*;
 
     fn generate_signing_key() -> SigningKey {
-        use rand::RngCore;
+        use rand::TryRngCore;
         let mut rng = rand::rngs::OsRng;
         let mut sk = [0u8; 32];
-        rng.fill_bytes(&mut sk);
+        rng.try_fill_bytes(&mut sk)
+            .expect("OsRng must succeed for cryptographic key generation");
         SigningKey::from_bytes(&sk)
     }