fix: avoid embedded opencode port conflicts

tests/runtime/opencode.test.ts

@@ -1,8 +1,12 @@
 import { describe, expect, it } from "bun:test";
 import { type OpencodeClient } from "@opencode-ai/sdk/v2";
+import { createServer } from "node:net";
 
 import { config } from "../../src/config.js";
-import { OpencodeRuntimeAdapter } from "../../src/runtime/opencode.js";
+import {
+  OpencodeRuntimeAdapter,
+  resolveEmbeddedOpencodePort,
+} from "../../src/runtime/opencode.js";
 
 const createRuntimeAdapter = (
   messages: unknown[],
@@ -87,6 +91,32 @@ describe("OpencodeRuntimeAdapter.ensureClient", () => {
   });
 });
 
+describe("resolveEmbeddedOpencodePort", () => {
+  it("falls back to a dynamic port when the configured port is occupied", async () => {
+    const server = createServer();
+    await new Promise<void>((resolve, reject) => {
+      server.once("error", reject);
+      server.listen(0, "127.0.0.1", resolve);
+    });
+
+    const address = server.address();
+    if (!address || typeof address === "string") {
+      server.close();
+      throw new Error("expected a TCP server address");
+    }
+
+    try {
+      await expect(
+        resolveEmbeddedOpencodePort("127.0.0.1", address.port),
+      ).resolves.toBe(0);
+    } finally {
+      await new Promise<void>((resolve, reject) => {
+        server.close((error) => (error ? reject(error) : resolve()));
+      });
+    }
+  });
+});
+
 describe("OpencodeRuntimeAdapter.warmup", () => {
   it("initializes the project session and model tools before reporting ready", async () => {
     const calls: string[] = [];