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/gemini.lua

@@ -0,0 +1,89 @@
+local adapter = {}
+
+adapter.name = "gemini"
+adapter.version = "2.0.0"
+adapter.endpoint = "/v1/models"
+adapter.headers = {}
+
+-- Gemini API: POST /v1/models/{model}:generateContent
+-- Auth: API key in query param ?key=XXX or Authorization: Bearer XXX
+function adapter.transform_request(raw_body)
+    local ok, req = pcall(json.decode, raw_body)
+    if not ok then return raw_body end
+
+    local contents = {}
+    for _, m in ipairs(req.messages or {}) do
+        table.insert(contents, {
+            role = (m.role == "assistant") and "model" or m.role,
+            parts = { { text = m.content } }
+        })
+    end
+
+    local gemini_req = {
+        contents = contents,
+        generationConfig = {
+            temperature = req.temperature or 0.7,
+            maxOutputTokens = req.max_tokens or 4096,
+        }
+    }
+
+    if req.stream then
+        gemini_req.stream = true
+    end
+
+    return json.encode(gemini_req)
+end
+
+-- Gemini 的 endpoint 动态拼接：/v1/models/{model}:generateContent
+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.usageMetadata then
+        unified.token_usage.prompt = resp.usageMetadata.promptTokenCount or 0
+        unified.token_usage.completion = resp.usageMetadata.candidatesTokenCount or 0
+        unified.token_usage.total = resp.usageMetadata.totalTokenCount or 0
+    end
+
+    if resp.candidates and #resp.candidates > 0 then
+        local cand = resp.candidates[1]
+        if cand.content and cand.content.parts then
+            for _, part in ipairs(cand.content.parts) do
+                if part.text then
+                    unified.content = unified.content .. part.text
+                end
+            end
+        end
+        if cand.finishReason then
+            unified.finish_reason = cand.finishReason
+        end
+    end
+
+    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 not chunk.candidates or #chunk.candidates == 0 then return "" end
+    local cand = chunk.candidates[1]
+    local content = ""
+    if cand.content and cand.content.parts then
+        for _, part in ipairs(cand.content.parts) do
+            content = content .. (part.text or "")
+        end
+    end
+    return json.encode({
+        content = content,
+        done = (cand.finishReason ~= nil)
+    })
+end
+
+return adapter