fix: imagegen MCP env 必须写死本地网关 base URL

首轮生产验收失败根因：_prepare_tenant 只把 IMAGE_GEN_* 透传
进 MCP env，没写 IMAGE_GEN_BASE_URL，也没带 SUB2API_BASE_URL，
于是 imagegen.py 回落到公网 sub2.diante.me，而它不在
Tinyproxy 白名单里，API 调用 403 Filtered。
修法：worker 在写 MCP env 前用 imagegen.build_mcp_env 算好
base（显式 IMAGE_GEN_BASE_URL > SUB2API_BASE_URL），并把
网关 host 加进 NO_PROXY；组装逻辑抽成可单测 helper，新增
2 项回归测试（本地网关解析、显式 base 优先）。

cloud/imagegen.py

@@ -306,3 +306,36 @@ def ensure_imagegen_mcp_env(codex_home_dir, env):
         return True
     except Exception:
         return False
+
+
+def build_mcp_env(worker_environ, proxy_env=None):
+    """Build the [mcp_servers.imagegen.env] dict from worker env.
+
+    The MCP process only sees config.toml env, never the worker's
+    own env, so IMAGE_GEN_BASE_URL must be resolved HERE (worker
+    IMAGE_GEN_BASE_URL, else SUB2API_BASE_URL, else absent and
+    imagegen.py falls back to the public relay domain -- which is
+    NOT on the Tinyproxy whitelist, so a missing base here caused
+    a 403 Filtered in the first production verification, 2026-10-30).
+    proxy_env supplies HTTP(S)_PROXY for downloading the returned
+    image URL; NO_PROXY gains the gateway host for direct API calls.
+    Never injects the task user's gateway key.
+    """
+    env = dict(proxy_env) if proxy_env else {}
+    for key in ("IMAGE_GEN_BASE_URL", "IMAGE_GEN_API_KEY",
+                "IMAGE_GEN_MODEL"):
+        if worker_environ.get(key):
+            env[key] = worker_environ[key]
+    base = (worker_environ.get("IMAGE_GEN_BASE_URL", "")
+            or worker_environ.get("SUB2API_BASE_URL", ""))
+    if base:
+        env["IMAGE_GEN_BASE_URL"] = base
+    host = base.split("://", 1)[-1].split("/", 1)[0]
+    host = host.split(":", 1)[0]
+    parts = [p for p in env.get("NO_PROXY", "").split(",") if p]
+    for item in ("localhost", "127.0.0.1", host):
+        if item and item not in parts:
+            parts.append(item)
+    env["NO_PROXY"] = ",".join(parts)
+    env["no_proxy"] = ",".join(parts)
+    return env