fix(opencodego): inject empty reasoning_content on tool-calling turns

v1.5.4 stopped stripping reasoning_content, which fixes clients that send
it — but most agent clients (pi included) never store or replay their
reasoning, keeping only the tool call. OpenCode Go validates the field on
any assistant turn that carries tool_calls and rejects the whole request:

  400 invalid_request_error: The `reasoning_content` in the thinking mode
  must be passed back to the API.

Verified against the live endpoint that an EMPTY string satisfies the
check, so the adapter now fills in "" when a tool-calling assistant turn
has no reasoning_content. Nothing is fabricated: the reasoning shown to
the client is still exactly what the upstream returned for that turn.

Measured: with a tool_call + tool_result history and no reasoning_content,
all 25 configured Go models returned 400 before and all 25 answer
correctly now.

Test: TestOpenCodeGoVsZenReasoning also pins that a plain assistant turn
(no tool calls) must NOT gain the field.

internal/lua/adapters/opencodego.lua

@@ -79,6 +79,17 @@ function adapter.transform_request(raw_body)
             -- 上游直接 400（"The `reasoning_content` in the thinking mode must
             -- be passed back to the API"），agent 的每一轮都会失败。
             -- 注意：与 opencodezen.lua 的行为**相反**，不要在这里剥。
+            --
+            -- 但绝大多数客户端（pi 等）根本不保存也不回传 reasoning，只保留
+            -- 工具调用本身。上游只在“带 tool_calls 的助手轮”上校验这个字段，
+            -- 实测**空串即可通过校验**，所以缺省时补空串：既满足上游的
+            -- 一致性要求，又不伪造任何推理内容（用户看到的 reasoning 仍然是
+            -- 上游本轮真实返回的）。
+            if msg.role == "assistant"
+                and type(msg.tool_calls) == "table" and #msg.tool_calls > 0
+                and msg.reasoning_content == nil then
+                msg.reasoning_content = ""
+            end
             local drop = false
             if type(msg.content) == "table" then
                 local parts = {}

internal/lua/toolcall_preservation_test.go

@@ -194,6 +194,32 @@ func TestOpenCodeGoVsZenReasoning(t *testing.T) {
 		t.Errorf("opencodezen must still keep the tool call: %s", zout)
 	}
 
+	// An assistant turn that carries tool_calls but no reasoning_content must
+	// get an empty one injected: Go validates that field on tool-calling turns
+	// and 400s when it is absent, while most clients never send it.
+	// Verified against the live endpoint that an empty string passes.
+	bare := `{"model":"m","messages":[
+		{"role":"user","content":"go"},
+		{"role":"assistant","content":"","tool_calls":[{"id":"c1","type":"function","function":{"name":"read","arguments":"{}"}}]},
+		{"role":"tool","tool_call_id":"c1","content":"r"}
+	]}`
+	bout, err := vm.Transform("opencodego", "transform_request", bare)
+	if err != nil {
+		t.Fatalf("opencodego bare: %v", err)
+	}
+	if !strings.Contains(bout, `"reasoning_content":""`) {
+		t.Errorf("opencodego must inject an empty reasoning_content on a tool-calling turn: %s", bout)
+	}
+	// A plain assistant turn (no tool calls) must NOT gain the field.
+	plainAsst := `{"model":"m","messages":[{"role":"user","content":"hi"},{"role":"assistant","content":"hello"}]}`
+	pout, err := vm.Transform("opencodego", "transform_request", plainAsst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(pout, "reasoning_content") {
+		t.Errorf("opencodego must not inject reasoning_content on a plain assistant turn: %s", pout)
+	}
+
 	// Both must still omit stream_options on non-streaming requests.
 	for _, a := range []string{"opencodego", "opencodezen"} {
 		out, err := vm.Transform(a, "transform_request", `{"model":"m","messages":[{"role":"user","content":"hi"}]}`)