Improve SSE tests (#1546)

## Summary
- support fixture-based SSE data in tests
- add helpers to load SSE JSON fixtures
- add table-driven SSE unit tests
- let integration tests use fixture loading
- fix clippy errors from format! calls

## Testing
- `cargo clippy --tests`
- `cargo test --workspace --exclude codex-linux-sandbox`


------
https://chatgpt.com/codex/tasks/task_i_68717468c3e48321b51c9ecac6ba0f09

codex-rs/core/tests/test_support.rs

@@ -21,3 +21,56 @@ pub fn load_default_config_for_test(codex_home: &TempDir) -> Config {
     )
     .expect("defaults for test should always succeed")
 }
+
+/// Builds an SSE stream body from a JSON fixture.
+///
+/// The fixture must contain an array of objects where each object represents a
+/// single SSE event with at least a `type` field matching the `event:` value.
+/// Additional fields become the JSON payload for the `data:` line. An object
+/// with only a `type` field results in an event with no `data:` section. This
+/// makes it trivial to extend the fixtures as OpenAI adds new event kinds or
+/// fields.
+pub fn load_sse_fixture(path: impl AsRef<std::path::Path>) -> String {
+    let events: Vec<serde_json::Value> =
+        serde_json::from_reader(std::fs::File::open(path).expect("read fixture"))
+            .expect("parse JSON fixture");
+    events
+        .into_iter()
+        .map(|e| {
+            let kind = e
+                .get("type")
+                .and_then(|v| v.as_str())
+                .expect("fixture event missing type");
+            if e.as_object().map(|o| o.len() == 1).unwrap_or(false) {
+                format!("event: {kind}\n\n")
+            } else {
+                format!("event: {kind}\ndata: {e}\n\n")
+            }
+        })
+        .collect()
+}
+
+/// Same as [`load_sse_fixture`], but replaces the placeholder `__ID__` in the
+/// fixture template with the supplied identifier before parsing. This lets a
+/// single JSON template be reused by multiple tests that each need a unique
+/// `response_id`.
+pub fn load_sse_fixture_with_id(path: impl AsRef<std::path::Path>, id: &str) -> String {
+    let raw = std::fs::read_to_string(path).expect("read fixture template");
+    let replaced = raw.replace("__ID__", id);
+    let events: Vec<serde_json::Value> =
+        serde_json::from_str(&replaced).expect("parse JSON fixture");
+    events
+        .into_iter()
+        .map(|e| {
+            let kind = e
+                .get("type")
+                .and_then(|v| v.as_str())
+                .expect("fixture event missing type");
+            if e.as_object().map(|o| o.len() == 1).unwrap_or(false) {
+                format!("event: {kind}\n\n")
+            } else {
+                format!("event: {kind}\ndata: {e}\n\n")
+            }
+        })
+        .collect()
+}