feat(provider): 端点机制去厂商化 —— 适配器声明 endpoint_stream

内核不再知道 Gemini 的流式动词。之前 provider.ChatURL 对
":generateContent" 做字符串替换（注释自认 "Gemini is the only such
adapter"），把厂商协议知识放进了调度器。

现在适配器自己声明：

    adapter.endpoint_stream = "/v1beta/models/{model}:streamGenerateContent"

内核只比较两个不透明字符串，四级优先级：

    source.endpoint_stream > source.endpoint > adapter.endpoint_stream
                                          > adapter.endpoint

- staticInfo.endpointStream + VM.StreamEndpoint() 编译期提取
- config.Source.StreamEndpoint（yaml: endpoint_stream）
- gemini.lua 声明 endpoint_stream
- 内核代码 generateContent 计数归零（仅注释/适配器持有）

首版实现踩了一个回归并当场修掉：源自定义 endpoint 时适配器的 stream
endpoint 会错误覆盖用户的显式路径。TestEndpointPrecedence 把四级优先级
钉死，防止再犯。

验证：go test -tags luajit ./... 全绿；golden 字节级未漂移。

internal/config/config.go

@@ -73,15 +73,20 @@ type Model struct {
 
 // Source describes a single upstream LLM provider.
 type Source struct {
-	Name          string            `yaml:"name" json:"name"`
-	BaseURL       string            `yaml:"base_url" json:"base_url"`
-	APIKey        string            `yaml:"api_key" json:"api_key"`
-	APIKeyEnv     string            `yaml:"api_key_env,omitempty" json:"-"` // reference to an env var holding the key (overrides api_key)
-	Adapter       string            `yaml:"adapter" json:"adapter"`
-	Endpoint      string            `yaml:"endpoint" json:"endpoint,omitempty"`             // chat endpoint override
-	ImageEndpoint string            `yaml:"image_endpoint" json:"image_endpoint,omitempty"` // image endpoint override
-	Models        []Model           `yaml:"models" json:"models"`
-	Headers       map[string]string `yaml:"headers" json:"headers,omitempty"`
+	Name      string `yaml:"name" json:"name"`
+	BaseURL   string `yaml:"base_url" json:"base_url"`
+	APIKey    string `yaml:"api_key" json:"api_key"`
+	APIKeyEnv string `yaml:"api_key_env,omitempty" json:"-"` // reference to an env var holding the key (overrides api_key)
+	Adapter   string `yaml:"adapter" json:"adapter"`
+	Endpoint  string `yaml:"endpoint" json:"endpoint,omitempty"` // chat endpoint override
+	// StreamEndpoint overrides the CHAT endpoint for streaming requests. Most
+	// upstreams stream on the same path; Gemini is the exception (same path,
+	// different verb). Declaring it here keeps that knowledge out of the kernel,
+	// which only compares opaque strings.
+	StreamEndpoint string            `yaml:"endpoint_stream" json:"endpoint_stream,omitempty"`
+	ImageEndpoint  string            `yaml:"image_endpoint" json:"image_endpoint,omitempty"` // image endpoint override
+	Models         []Model           `yaml:"models" json:"models"`
+	Headers        map[string]string `yaml:"headers" json:"headers,omitempty"`
 	// ProxyURL routes this source's HTTP(S) traffic through an explicit proxy
 	// (e.g. http://127.0.0.1:7890). Empty = direct connection. Needed for
 	// upstreams behind geo/IP blocks that only respond through a proxy.
@@ -373,13 +378,14 @@ type RuntimeConfig struct {
 // name and api_key) so the WebUI can spin up multiple key-bearing sources
 // from one shared template.
 type SourceTemplate struct {
-	Name          string            `json:"name"`
-	BaseURL       string            `json:"base_url"`
-	Adapter       string            `json:"adapter"`
-	Endpoint      string            `json:"endpoint,omitempty"`
-	ImageEndpoint string            `json:"image_endpoint,omitempty"`
-	Models        []Model           `json:"models"`
-	Headers       map[string]string `json:"headers,omitempty"`
+	Name           string            `json:"name"`
+	BaseURL        string            `json:"base_url"`
+	Adapter        string            `json:"adapter"`
+	Endpoint       string            `json:"endpoint,omitempty"`
+	StreamEndpoint string            `json:"endpoint_stream,omitempty"`
+	ImageEndpoint  string            `json:"image_endpoint,omitempty"`
+	Models         []Model           `json:"models"`
+	Headers        map[string]string `json:"headers,omitempty"`
 	// ProxyURL routes this source's HTTP(S) traffic through an explicit proxy.
 	ProxyURL      string                 `json:"proxy_url,omitempty"`
 	Meta          map[string]interface{} `json:"meta,omitempty"`
@@ -393,18 +399,18 @@ type SourceTemplate struct {
 // (full management) or "user" (sees only its own key); Models is the allowed
 // model scope with per-model token quota (0 = unlimited).
 type GWKey struct {
-	Key   string `yaml:"key" json:"key"`
-	Role  string `yaml:"role" json:"role"`
-	Name  string `yaml:"name,omitempty" json:"name,omitempty"`
+	Key  string `yaml:"key" json:"key"`
+	Role string `yaml:"role" json:"role"`
+	Name string `yaml:"name,omitempty" json:"name,omitempty"`
 	// Auto is this key's own AUTO scheduling chain. It overrides the global
 	// cfg.Auto for requests authenticated with this key; nil (or an empty
 	// list) means "inherit the global chain", so keys created before per-key
 	// chains existed keep working untouched.
-	Auto           []ModelScope `yaml:"auto,omitempty" json:"auto,omitempty"`
-	Models         []ModelScope `yaml:"models,omitempty" json:"models,omitempty"`
-	Note           string       `yaml:"note,omitempty" json:"note,omitempty"`
-	CreatedAt      int64        `yaml:"created_at,omitempty" json:"created_at,omitempty"`
-	Seed           bool         `yaml:"seed,omitempty" json:"seed,omitempty"` // true if migrated from config gateway_keys
+	Auto      []ModelScope `yaml:"auto,omitempty" json:"auto,omitempty"`
+	Models    []ModelScope `yaml:"models,omitempty" json:"models,omitempty"`
+	Note      string       `yaml:"note,omitempty" json:"note,omitempty"`
+	CreatedAt int64        `yaml:"created_at,omitempty" json:"created_at,omitempty"`
+	Seed      bool         `yaml:"seed,omitempty" json:"seed,omitempty"` // true if migrated from config gateway_keys
 }
 
 // HasOwnAuto reports whether the key declares its own AUTO chain rather than