fix(tests): align WebSocket connection module layout

Move production and test connection modules to connection/mod.rs so the
heartbeat submodule follows the workspace directory module convention.
Keep the production and integration test layouts mirrored, and update the
frozen file manifests to include the heartbeat regression tests.

Validation: gateway application tests (36 passed), all gateway-api tests
(263 passed), workspace rustfmt, and strict gateway-api Clippy with all
targets and features.

Co-authored-by: Codex <noreply@openai.com>

backend/crates/gateway-api/tests/architecture.rs

@@ -67,7 +67,7 @@ fn source_tree_should_match_frozen_machine_manifest() {
         "src/openai/responses/mod.rs",
         "src/openai/responses/request.rs",
         "src/openai/responses/response.rs",
-        "src/openai/responses/websocket/connection.rs",
+        "src/openai/responses/websocket/connection/mod.rs",
         "src/openai/responses/websocket/forward.rs",
         "src/openai/responses/websocket/mod.rs",
         "src/openai/responses/websocket/protocol.rs",
@@ -110,7 +110,8 @@ fn test_tree_should_match_frozen_rust_mirror() {
         "tests/openai/provider_endpoint.rs",
         "tests/openai/responses/http.rs",
         "tests/openai/responses/mod.rs",
-        "tests/openai/responses/websocket/connection.rs",
+        "tests/openai/responses/websocket/connection/heartbeat.rs",
+        "tests/openai/responses/websocket/connection/mod.rs",
         "tests/openai/responses/websocket/mod.rs",
         "tests/openai/responses/websocket/protocol.rs",
         "tests/openai/router.rs",