fix(opencode): never drop an assistant turn that carries tool_calls

An agent client (pi) serialises an assistant turn whose content is only
[thinking, toolCall] as content:[] with tool_calls. The multimodal-strip
pass treated an empty content array as 'nothing left, drop the whole
message' and discarded the tool_calls with it.

The next message is that call's tool result, so it arrived orphaned: the
model saw a result for a call it had never made and re-issued the same
call on every turn — an endless repeated-tool-call loop. Reproduced
against a capture sink: content:[] lost tool_calls, while content:"" and
content:null kept them.

Only messages with neither usable content NOR a tool call now get
dropped. Content that collapses to empty but still has tool_calls or a
tool_call_id is emitted as "" instead.

Test: TestOpenCodeKeepsToolCallWithEmptyContent (plus a negative control
that an image-only message without tool calls is still dropped).

internal/lua/adapters/opencode.lua

@@ -34,8 +34,10 @@ end
 
 -- OpenAI /chat/completions format (pass-through, strip provider-specific fields)
 -- zen 上游 schema 只接受 text content part（无视觉/音频能力）：多模态 part
--- （image_url / input_audio / file 等）一律剥离；因此失去全部 content 的
--- 消息整条丢弃，避免上游 "unknown variant `image_url`, expected `text`"。
+-- （image_url / input_audio / file 等）一律剥离。剥离后 content 变空的消息
+-- 若不再携带 tool_calls / tool_call_id 才整条丢弃（避免上游
+-- "unknown variant `image_url`, expected `text`"）；带工具调用的必须保留，
+-- 否则会把紧随其后的 tool 结果变成孤儿，模型会反复重发同一个调用。
 -- zen 上游角色白名单只有 system / user / assistant / tool / latest_reminder：
 -- OpenAI 的 developer（及 function 等）不在其中，直接透传会触发上游
 -- "unknown variant `developer`, expected one of ..." 错误；统一归一化为 system。
@@ -72,7 +74,24 @@ function adapter.transform_request(raw_body)
                     end
                 end
                 if #parts == 0 then
-                    drop = true
+                    -- Content collapsed to nothing after stripping unsupported
+                    -- parts. A message that still carries a tool call must
+                    -- NEVER be dropped: the very next message is its tool
+                    -- result, and dropping the call orphans that result. The
+                    -- model then sees a result for a call it never made and
+                    -- re-issues the same tool call on every turn (observed as
+                    -- an infinite "repeated tool call" loop).
+                    --
+                    -- This is the common shape for agent clients: an assistant
+                    -- turn whose content is only [thinking, toolCall] serialises
+                    -- to content:[] with tool_calls — exactly the case that used
+                    -- to vanish here. Emit "" instead, which zen accepts.
+                    if (type(msg.tool_calls) == "table" and #msg.tool_calls > 0)
+                        or msg.tool_call_id ~= nil then
+                        msg.content = ""
+                    else
+                        drop = true
+                    end
                 else
                     msg.content = parts
                 end