Omit empty app-server instruction overrides (#17258)

## Summary
- omit serialized Responses instructions when an app-server base
instruction override is empty
- skip empty developer instruction messages and add v2 coverage for the
empty-override request shape

## Validation
- just fmt
- git diff --check

codex-rs/codex-api/src/common.rs

@@ -24,6 +24,7 @@ pub const WS_REQUEST_HEADER_TRACESTATE_CLIENT_METADATA_KEY: &str = "ws_request_h
 pub struct CompactionInput<'a> {
     pub model: &'a str,
     pub input: &'a [ResponseItem],
+    #[serde(skip_serializing_if = "str::is_empty")]
     pub instructions: &'a str,
     pub tools: Vec<Value>,
     pub parallel_tool_calls: bool,
@@ -153,6 +154,7 @@ impl From<VerbosityConfig> for OpenAiVerbosity {
 #[derive(Debug, Serialize, Clone, PartialEq)]
 pub struct ResponsesApiRequest {
     pub model: String,
+    #[serde(skip_serializing_if = "String::is_empty")]
     pub instructions: String,
     pub input: Vec<ResponseItem>,
     pub tools: Vec<serde_json::Value>,
@@ -198,6 +200,7 @@ impl From<&ResponsesApiRequest> for ResponseCreateWsRequest {
 #[derive(Debug, Serialize)]
 pub struct ResponseCreateWsRequest {
     pub model: String,
+    #[serde(skip_serializing_if = "String::is_empty")]
     pub instructions: String,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub previous_response_id: Option<String>,