fix(api): tolerate null tool_calls in OpenAI-compat stream delta chunks

Some OpenAI-compatible providers emit 'tool_calls: null' in streaming
delta chunks instead of omitting the field or using an empty array:

  "delta": {"content":"","function_call":null,"tool_calls":null}

serde's #[serde(default)] only handles absent keys — an explicit null
value still fails deserialization with:
  'invalid type: null, expected a sequence'

Fix: replace #[serde(default)] with a custom deserializer helper
deserialize_null_as_empty_vec() that maps null -> Vec::default(),
keeping the existing absent-key default behaviour.

Regression test added: delta_with_null_tool_calls_deserializes_as_empty_vec
uses the exact provider response shape from gaebal-gajae's repro (2026-04-09).

112 api lib tests pass. Fmt clean.

Companion to gaebal-gajae's local 448cf2c — independently reproduced
and landed on main.

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

@@ -255,6 +255,19 @@ impl OpenAiCompatClient {
 static JITTER_COUNTER: AtomicU64 = AtomicU64::new(0);
 
 /// Returns a random additive jitter in `[0, base]` to decorrelate retries
+/// Deserialize a JSON field as a `Vec<T>`, treating an explicit `null` value
+/// the same as a missing field (i.e. as an empty vector).
+/// Some OpenAI-compatible providers emit `"tool_calls": null` instead of
+/// omitting the field or using `[]`, which serde's `#[serde(default)]` alone
+/// does not tolerate — `default` only handles absent keys, not null values.
+fn deserialize_null_as_empty_vec<'de, D, T>(deserializer: D) -> Result<Vec<T>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+    T: serde::Deserialize<'de>,
+{
+    Ok(Option::<Vec<T>>::deserialize(deserializer)?.unwrap_or_default())
+}
+
 /// from multiple concurrent clients. Entropy is drawn from the nanosecond
 /// wall clock mixed with a monotonic counter and run through a splitmix64
 /// finalizer; adequate for retry jitter (no cryptographic requirement).
@@ -673,7 +686,7 @@ struct ChunkChoice {
 struct ChunkDelta {
     #[serde(default)]
     content: Option<String>,
-    #[serde(default)]
+    #[serde(default, deserialize_with = "deserialize_null_as_empty_vec")]
     tool_calls: Vec<DeltaToolCall>,
 }
 
@@ -1484,6 +1497,35 @@ mod tests {
         );
     }
 
+    /// Regression test: some OpenAI-compatible providers emit `"tool_calls": null`
+    /// in stream delta chunks instead of omitting the field or using `[]`.
+    /// Before the fix this produced: `invalid type: null, expected a sequence`.
+    #[test]
+    fn delta_with_null_tool_calls_deserializes_as_empty_vec() {
+        // Simulate the exact shape observed in the wild (gaebal-gajae repro 2026-04-09)
+        let json = r#"{
+            "content": "",
+            "function_call": null,
+            "refusal": null,
+            "role": "assistant",
+            "tool_calls": null
+        }"#;
+
+        use super::deserialize_null_as_empty_vec;
+        #[derive(serde::Deserialize, Debug)]
+        struct Delta {
+            content: Option<String>,
+            #[serde(default, deserialize_with = "deserialize_null_as_empty_vec")]
+            tool_calls: Vec<super::DeltaToolCall>,
+        }
+        let delta: Delta = serde_json::from_str(json)
+            .expect("delta with tool_calls:null must deserialize without error");
+        assert!(
+            delta.tool_calls.is_empty(),
+            "tool_calls:null must produce an empty vec, not an error"
+        );
+    }
+
     #[test]
     fn non_gpt5_uses_max_tokens() {
         // Older OpenAI models expect `max_tokens`; verify gpt-4o is unaffected.