fix(openai): suppress native image_generation tool for Codex models

In ChatGPT mode, build_response_request unconditionally pushed
{ "type": "image_generation" } onto the request's tool array. The
OpenAI Responses API rejects that tool for codex-family models
(gpt-5.x-codex*, including the [1m] long-context suffix variants)
with HTTP 400 ("unsupported tool image_generation for model …"),
which aborted the entire session for ChatGPT-mode codex users.

Add Self::supports_native_image_generation(model_id) returning false
for any model id whose lower-cased / [1m]-stripped form contains
"codex". Gate the tools.push site by that helper. Non-codex ChatGPT
models (gpt-5.x non-codex, gpt-4o, etc.) still receive the tool.

Two regression tests in src/provider/openai_tests/payloads.rs:
- test_chatgpt_payload_includes_native_image_generation_for_non_codex_models
- test_chatgpt_payload_excludes_native_image_generation_for_codex_models
  iterating gpt-5.3-codex, gpt-5.3-codex-spark, and gpt-5.3-codex-spark[1m].

Ports upstream PR 1jehuang#148.
Closes #115

Author: quangdang46

src/provider/openai.rs

@@ -702,6 +702,20 @@ impl OpenAIProvider {
         format!("{}/compact", Self::responses_url(credentials))
     }
 
+    /// Codex-family models (`gpt-5.x-codex*`, including the `[1m]` long-context
+    /// suffix variants) do not accept the native `image_generation` tool. The
+    /// OpenAI Responses API rejects requests with that tool attached and the
+    /// session aborts with a 400. Other ChatGPT-mode models (gpt-5.x non-codex,
+    /// gpt-4o, …) still support it, so we only suppress the tool for codex
+    /// model ids.
+    fn supports_native_image_generation(model_id: &str) -> bool {
+        let normalized = model_id
+            .strip_suffix("[1m]")
+            .unwrap_or(model_id)
+            .to_ascii_lowercase();
+        !normalized.contains("codex")
+    }
+
     #[expect(
         clippy::too_many_arguments,
         reason = "request construction threads explicit per-request OpenAI settings without hidden state"
@@ -720,7 +734,7 @@ impl OpenAIProvider {
         native_compaction_threshold: Option<usize>,
     ) -> Value {
         let mut tools = api_tools.to_vec();
-        if is_chatgpt_mode {
+        if is_chatgpt_mode && Self::supports_native_image_generation(model_id) {
             tools.push(serde_json::json!({ "type": "image_generation" }));
         }
 

src/provider/openai_tests/payloads.rs

@@ -17,6 +17,70 @@ fn test_build_response_request_includes_stream_for_http() {
     assert_eq!(request["store"], serde_json::json!(false));
 }
 
+#[test]
+fn test_chatgpt_payload_includes_native_image_generation_for_non_codex_models() {
+    // Regression for issue #115: ChatGPT mode adds a native `image_generation`
+    // tool. Non-codex models (gpt-5.x, gpt-4o, …) accept it and benefit.
+    let request = OpenAIProvider::build_response_request(
+        "gpt-5.5",
+        "system".to_string(),
+        &[],
+        &[],
+        true,
+        Some(DEFAULT_MAX_OUTPUT_TOKENS),
+        None,
+        None,
+        None,
+        None,
+        None,
+    );
+
+    assert!(
+        request["tools"]
+            .as_array()
+            .expect("tools array")
+            .iter()
+            .any(|tool| tool["type"] == "image_generation"),
+        "non-codex models should still receive image_generation in ChatGPT mode"
+    );
+}
+
+#[test]
+fn test_chatgpt_payload_excludes_native_image_generation_for_codex_models() {
+    // Regression for issue #115: codex-family models reject the native
+    // image_generation tool with a 400 from the OpenAI Responses API. Suppress
+    // it for any model id whose normalized form contains "codex", including
+    // the `[1m]` long-context suffix variant.
+    for model in [
+        "gpt-5.3-codex",
+        "gpt-5.3-codex-spark",
+        "gpt-5.3-codex-spark[1m]",
+    ] {
+        let request = OpenAIProvider::build_response_request(
+            model,
+            "system".to_string(),
+            &[],
+            &[],
+            true,
+            Some(DEFAULT_MAX_OUTPUT_TOKENS),
+            None,
+            None,
+            None,
+            None,
+            None,
+        );
+
+        assert!(
+            request["tools"]
+                .as_array()
+                .expect("tools array")
+                .iter()
+                .all(|tool| tool["type"] != "image_generation"),
+            "{model} should not receive the unsupported native image_generation tool"
+        );
+    }
+}
+
 #[test]
 fn test_websocket_payload_strips_stream_and_background() {
     let mut request = OpenAIProvider::build_response_request(