[apply_patch] freeform apply_patch tool (#2576)

## Summary
GPT-5 introduced the concept of [custom
tools](https://platform.openai.com/docs/guides/function-calling#custom-tools),
which allow the model to send a raw string result back, simplifying
json-escape issues. We are migrating gpt-5 to use this by default.

However, gpt-oss models do not support custom tools, only normal
functions. So we keep both tool definitions, and provide whichever one
the model family supports.

## Testing
- [x] Tested locally with various models
- [x] Unit tests pass

codex-rs/core/src/client_common.rs

@@ -49,13 +49,14 @@ impl Prompt {
             .unwrap_or(BASE_INSTRUCTIONS);
         let mut sections: Vec<&str> = vec![base];
 
-        // When there are no custom instructions, add apply_patch if either:
-        // - the model needs special instructions, or
+        // When there are no custom instructions, add apply_patch_tool_instructions if either:
+        // - the model needs special instructions (4.1), or
         // - there is no apply_patch tool present
-        let is_apply_patch_tool_present = self
-            .tools
-            .iter()
-            .any(|t| matches!(t, OpenAiTool::Function(f) if f.name == "apply_patch"));
+        let is_apply_patch_tool_present = self.tools.iter().any(|tool| match tool {
+            OpenAiTool::Function(f) => f.name == "apply_patch",
+            OpenAiTool::Freeform(f) => f.name == "apply_patch",
+            _ => false,
+        });
         if self.base_instructions_override.is_none()
             && (model.needs_special_apply_patch_instructions || !is_apply_patch_tool_present)
         {