fix(agent): use canonical assistant history format in subagent_runner (#606)

build_native_assistant_payload in subagent_runner.rs serialised the
assistant tool-call message using {"text": …, "tool_calls": [{
"type":"function","function":{…}}]} — two mismatches vs the format
parse::build_native_assistant_history produces and the backend
jinja template expects:

  1. "text" instead of "content" — the downstream convert_messages
     parser looks for "content" to detect assistant-with-tool-calls
     messages; "text" is not recognised, so the message is treated
     as a plain assistant message.
  2. Nested {"type":"function","function":{"name":…,"arguments":…}}
     wrappers instead of flat {"id":…,"name":…,"arguments":…}.

Result: every sub-agent that made a tool call and entered iteration
1+ hit a 400 from the backend: "jinja template rendering failed.
Message has tool role, but there was no previous assistant message
with a tool call!" — the backend saw a role=tool message without a
recognised assistant-with-tool-calls predecessor.

The fix replaces the broken inline copy with a direct call to
parse::build_native_assistant_history (the same serialiser
tool_loop.rs uses successfully for the orchestrator's tool calls).
The dead build_native_assistant_payload function is removed.

Introduced in PR #474 (6465f3d3, 2026-04-09). Latent since then
because the orchestrator self-heals by retrying via other agents.

E2E verified: skills_agent with gmail toolkit now progresses through
iterations 0→1→2 successfully (previously died at iteration 1 with
the 400 error every time).

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/openhuman/agent/harness/subagent_runner.rs

@@ -728,8 +728,15 @@ async fn run_inner_loop(
 
         // Persist assistant message with the original tool_calls payload so
         // subsequent role=tool messages can reference call ids correctly.
+        // Uses the canonical serialiser from `parse` — the old inline
+        // `build_native_assistant_payload` used `"text"` instead of
+        // `"content"` and nested `{"type":"function","function":{…}}`
+        // wrappers instead of flat `{id, name, arguments}`, which caused
+        // 400 errors from the backend jinja template ("Message has tool
+        // role, but there was no previous assistant message with a tool
+        // call!").
         let assistant_history_content =
-            build_native_assistant_payload(&response_text, &native_calls);
+            super::parse::build_native_assistant_history(&response_text, &native_calls);
         history.push(ChatMessage::assistant(assistant_history_content));
 
         // Execute each call, append role=tool messages.
@@ -779,31 +786,6 @@ async fn run_inner_loop(
     Err(SubagentRunError::MaxIterationsExceeded(max_iterations))
 }
 
-fn build_native_assistant_payload(text: &str, tool_calls: &[ToolCall]) -> String {
-    // Mirror the existing native-tool-call serialisation pattern used by
-    // `agent::loop_::parse::build_native_assistant_history`. We inline a
-    // small subset here to avoid an inter-module dep cycle.
-    let calls_json: Vec<serde_json::Value> = tool_calls
-        .iter()
-        .map(|call| {
-            serde_json::json!({
-                "id": call.id,
-                "type": "function",
-                "function": {
-                    "name": call.name,
-                    "arguments": call.arguments,
-                },
-            })
-        })
-        .collect();
-
-    let payload = serde_json::json!({
-        "text": text,
-        "tool_calls": calls_json,
-    });
-    payload.to_string()
-}
-
 fn parse_tool_arguments(arguments: &str) -> serde_json::Value {
     serde_json::from_str(arguments)
         .unwrap_or_else(|_| serde_json::Value::Object(Default::default()))