feat(api): add reasoning_effort field to MessageRequest and OpenAI-compat path

Users of OpenAI-compatible reasoning models (o4-mini, o3, deepseek-r1,
etc.) had no way to control reasoning effort — the field was missing from
MessageRequest and never emitted in the request body.

Changes:
- Add `reasoning_effort: Option<String>` to `MessageRequest` in types.rs
  - Annotated with skip_serializing_if = "Option::is_none" for clean JSON
  - Accepted values: "low", "medium", "high" (passed through verbatim)
- In `build_chat_completion_request`, emit `"reasoning_effort"` when set
- Two unit tests:
  - `reasoning_effort_is_included_when_set`: o4-mini + "high" → field present
  - `reasoning_effort_omitted_when_not_set`: gpt-4o, no field → absent

Existing callers use `..Default::default()` and are unaffected.
One struct-literal test that listed all fields explicitly updated with
`reasoning_effort: None`.

The CLI flag to expose this to users is a follow-up (ROADMAP #34 partial).
This commit lands the foundational API-layer plumbing needed for that.

Partial ROADMAP #34.

rust/crates/api/src/providers/openai_compat.rs

@@ -801,6 +801,10 @@ fn build_chat_completion_request(request: &MessageRequest, config: OpenAiCompatC
             payload["stop"] = json!(stop);
         }
     }
+    // reasoning_effort for OpenAI-compatible reasoning models (o4-mini, o3, etc.)
+    if let Some(effort) = &request.reasoning_effort {
+        payload["reasoning_effort"] = json!(effort);
+    }
 
     payload
 }
@@ -1216,6 +1220,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn reasoning_effort_is_included_when_set() {
+        let payload = build_chat_completion_request(
+            &MessageRequest {
+                model: "o4-mini".to_string(),
+                max_tokens: 1024,
+                messages: vec![InputMessage::user_text("think hard")],
+                reasoning_effort: Some("high".to_string()),
+                ..Default::default()
+            },
+            OpenAiCompatConfig::openai(),
+        );
+        assert_eq!(payload["reasoning_effort"], json!("high"));
+    }
+
+    #[test]
+    fn reasoning_effort_omitted_when_not_set() {
+        let payload = build_chat_completion_request(
+            &MessageRequest {
+                model: "gpt-4o".to_string(),
+                max_tokens: 64,
+                messages: vec![InputMessage::user_text("hello")],
+                ..Default::default()
+            },
+            OpenAiCompatConfig::openai(),
+        );
+        assert!(payload.get("reasoning_effort").is_none());
+    }
+
     #[test]
     fn openai_streaming_requests_include_usage_opt_in() {
         let payload = build_chat_completion_request(
@@ -1333,6 +1366,7 @@ mod tests {
             frequency_penalty: Some(0.5),
             presence_penalty: Some(0.3),
             stop: Some(vec!["\n".to_string()]),
+            reasoning_effort: None,
         };
         let payload = build_chat_completion_request(&request, OpenAiCompatConfig::openai());
         assert_eq!(payload["temperature"], 0.7);

rust/crates/api/src/types.rs

@@ -26,6 +26,11 @@ pub struct MessageRequest {
     pub presence_penalty: Option<f64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub stop: Option<Vec<String>>,
+    /// Reasoning effort level for OpenAI-compatible reasoning models (e.g. `o4-mini`).
+    /// Accepted values: `"low"`, `"medium"`, `"high"`. Omitted when `None`.
+    /// Silently ignored by backends that do not support it.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub reasoning_effort: Option<String>,
 }
 
 impl MessageRequest {