chore(vendor): import pi 1.3.0

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: 71023aec1546d6d677a65669c5665c0927ce0dd0

third_party/pi/packages/mcp/test/fixtures/stdio-server.mjs

@@ -0,0 +1,27 @@
+import { createInterface } from "node:readline";
+
+console.error("stdio fixture ready");
+const lines = createInterface({ input: process.stdin, crlfDelay: Infinity });
+
+for await (const line of lines) {
+	const message = JSON.parse(line);
+	if (!("id" in message)) continue;
+	let result;
+	if (message.method === "initialize") {
+		result = {
+			protocolVersion: "2025-06-18",
+			capabilities: { tools: {} },
+			serverInfo: { name: "stdio-fixture", version: "1.0.0" },
+		};
+	} else if (message.method === "tools/list") {
+		result = { tools: [{ name: "echo", inputSchema: { type: "object" } }] };
+	} else if (message.method === "tools/call") {
+		result = { content: [{ type: "text", text: String(message.params.arguments.text) }] };
+	} else if (message.method === "ping") {
+		result = {};
+	} else {
+		process.stdout.write(`${JSON.stringify({ jsonrpc: "2.0", id: message.id, error: { code: -32601, message: "not found" } })}\n`);
+		continue;
+	}
+	process.stdout.write(`${JSON.stringify({ jsonrpc: "2.0", id: message.id, result })}\n`);
+}