feat(translator): lossless passthrough via CLI tool + provider pairing

Add clientDetector utility to identify CLI tools (Claude Code, Gemini CLI,
Antigravity, Codex) from request headers. When the CLI tool and provider
are a native pair, skip all translation — only swap model and Bearer token.

Made-with: Cursor

open-sse/utils/clientDetector.js

@@ -0,0 +1,53 @@
+/**
+ * Detect CLI tool identity from request headers/body.
+ * Used to determine if a request can be passed through losslessly.
+ */
+
+// Map of CLI tool identifiers to provider IDs they are "native" to
+const NATIVE_PAIRS = {
+  "claude":      ["claude", "anthropic"],
+  "gemini-cli":  ["gemini-cli"],
+  "antigravity": ["antigravity"],
+  "codex":       ["codex"],
+};
+
+/**
+ * Detect which CLI tool is making the request.
+ * Returns one of: "claude" | "gemini-cli" | "antigravity" | "codex" | null
+ * @param {object} headers - Lowercase header key/value object
+ * @param {object} body    - Parsed request body
+ */
+export function detectClientTool(headers = {}, body = {}) {
+  const ua = (headers["user-agent"] || "").toLowerCase();
+  const xApp = (headers["x-app"] || "").toLowerCase();
+
+  // Antigravity: detected via body field (not header)
+  if (body.userAgent === "antigravity") return "antigravity";
+
+  // Claude Code / Claude CLI
+  if (ua.includes("claude-cli") || ua.includes("claude-code") || xApp === "cli") return "claude";
+
+  // Gemini CLI
+  if (ua.includes("gemini-cli")) return "gemini-cli";
+
+  // Codex CLI
+  if (ua.includes("codex-cli")) return "codex";
+
+  return null;
+}
+
+/**
+ * Check if this CLI tool + provider pair should be passed through losslessly.
+ * @param {string|null} clientTool - Result of detectClientTool()
+ * @param {string} provider        - Provider ID (e.g. "claude", "gemini-cli")
+ */
+export function isNativePassthrough(clientTool, provider) {
+  if (!clientTool) return false;
+  const nativeProviders = NATIVE_PAIRS[clientTool];
+  if (!nativeProviders) return false;
+  // Support anthropic-compatible-* variants
+  const normalizedProvider = provider.startsWith("anthropic-compatible")
+    ? "anthropic"
+    : provider;
+  return nativeProviders.includes(normalizedProvider);
+}