feat(security): harden gateway boundaries and usage policies

Consolidate subscription usage policy enforcement, privacy-safe persistence, and gateway security hardening into one reviewable change.

Includes bounded HTTP and execution envelopes, header and protocol guards, DNS and relay validation, authentication and secret projection hardening, secure backup/install paths, and regression coverage.

crates/aether-provider/transport/src/windsurf.rs

@@ -3,6 +3,7 @@ use std::collections::BTreeMap;
 use serde_json::{json, Value};
 use uuid::Uuid;
 
+use super::headers::{declared_connection_header_names, remove_declared_connection_headers};
 use crate::rules::{
     apply_local_body_rules_with_request_headers, apply_local_header_rules_with_request_headers,
     body_rules_are_locally_supported, header_rules_are_locally_supported,
@@ -190,13 +191,16 @@ pub fn build_windsurf_cascade_headers(
     auth_value: &str,
     _upstream_is_stream: bool,
 ) -> Option<BTreeMap<String, String>> {
+    let declared_connection_headers = declared_connection_header_names(headers, &BTreeMap::new());
     let mut out = BTreeMap::new();
     for (name, value) in headers {
         let Ok(value) = value.to_str() else {
             continue;
         };
         let key = name.as_str().to_ascii_lowercase();
-        if should_skip_upstream_passthrough_header(&key) {
+        if should_skip_upstream_passthrough_header(&key)
+            || declared_connection_headers.contains(&key)
+        {
             continue;
         }
         let value = value.trim();
@@ -234,6 +238,7 @@ pub fn build_windsurf_cascade_headers(
     if !auth_header.is_empty() {
         out.insert(auth_header, auth_value.trim().to_string());
     }
+    remove_declared_connection_headers(&mut out, &declared_connection_headers);
     out.remove("content-length");
     Some(out)
 }