agent-vm: set SecretEntry.on_violation for msb 0.5.7 SDK

The 0.5.7 secrets config gained a per-entry on_violation override
(Option<ViolationAction>). The token-isolation test constructs a
SecretEntry literally, so it must set the new field. None = inherit
the collection-level default. Release build was unaffected (only test
code constructs SecretEntry); cargo test -p agent-vm now compiles.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/agent-vm/src/intercept_hook.rs

@@ -1612,6 +1612,7 @@ mod tests {
                     query_params: false,
                     body: false,
                 },
+                on_violation: None,
                 require_tls_identity: true,
             }],
             on_violation: Default::default(),