feat: ModelRouter — unified OpenAI-compatible multi-source LLM gateway

- Lua adapters per upstream (transform_request/response/stream_chunk, build_headers signing hooks)
- AUTO priority routing with per-model kind (chat/image), explicit source/model routing
- Per-source concurrency caps with queueing, exponential backoff, AUTO failover
- OpenAI-compatible API: chat completions, SSE streaming, image generations, models
- Gateway key auth, web UI for adapter/source management, runtime persistence
- e2e test running the real binary against mocked upstreams

internal/lua/adapters/anthropic.lua

@@ -0,0 +1,86 @@
+local adapter = {}
+
+adapter.name = "anthropic"
+adapter.version = "2.0.0"
+adapter.endpoint = "/v1/messages"
+adapter.headers = {
+    ["anthropic-version"] = "2023-06-01"
+}
+
+function adapter.transform_request(raw_body)
+    local ok, req = pcall(json.decode, raw_body)
+    if not ok then return raw_body end
+
+    local msgs = {}
+    local system = ""
+    for _, m in ipairs(req.messages or {}) do
+        if m.role == "system" then
+            system = system .. m.content .. "\n"
+        else
+            table.insert(msgs, { role = m.role, content = m.content })
+        end
+    end
+
+    local anthropic_req = {
+        model = req.model or "claude-sonnet-4-20250514",
+        max_tokens = req.max_tokens or 4096,
+        messages = msgs,
+        stream = req.stream or false,
+    }
+
+    if not req.disable_thinking then
+        anthropic_req.thinking = { type = "enabled", budget_tokens = 4096 }
+    end
+
+    if system ~= "" then
+        anthropic_req.system = system
+    end
+
+    return json.encode(anthropic_req)
+end
+
+function adapter.transform_response(raw_body)
+    local ok, resp = pcall(json.decode, raw_body)
+    if not ok then return raw_body end
+
+    local unified = {
+        content = "",
+        finish_reason = "",
+        token_usage = { prompt = 0, completion = 0, total = 0 }
+    }
+
+    if resp.usage then
+        unified.token_usage.prompt = resp.usage.input_tokens or 0
+        unified.token_usage.completion = resp.usage.output_tokens or 0
+        unified.token_usage.total = (resp.usage.input_tokens or 0) + (resp.usage.output_tokens or 0)
+    end
+
+    if resp.content and #resp.content > 0 then
+        for _, block in ipairs(resp.content) do
+            if block.type == "text" then
+                unified.content = unified.content .. (block.text or "")
+            end
+        end
+    end
+    unified.finish_reason = resp.stop_reason or ""
+
+    return json.encode(unified)
+end
+
+function adapter.transform_stream_chunk(raw_chunk)
+    local ok, chunk = pcall(json.decode, raw_chunk)
+    if not ok then return "" end
+    if chunk.type == "message_start" then return "" end
+    if chunk.type == "message_delta" then
+        return json.encode({ content = "", done = (chunk.delta and chunk.delta.stop_reason ~= nil) })
+    end
+    if chunk.type == "content_block_delta" and chunk.delta then
+        return json.encode({ content = chunk.delta.text or "", done = false })
+    end
+    if chunk.type == "message_stop" then
+        return json.encode({ content = "", done = true })
+    end
+    return ""
+end
+
+return adapter