fix(exec): resolve PATH key case-insensitively for Windows pathPrepend (#25399) (#31879)

Co-authored-by: Glucksberg <markuscontasul@gmail.com>

src/agents/bash-tools.test.ts

@@ -1,5 +1,6 @@
 import path from "node:path";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { applyPathPrepend, findPathKey } from "../infra/path-prepend.js";
 import { peekSystemEvents, resetSystemEventsForTest } from "../infra/system-events.js";
 import { captureEnv } from "../test-utils/env.js";
 import { getFinishedSession, resetProcessRegistryForTests } from "./bash-process-registry.js";
@@ -547,3 +548,57 @@ describe("exec PATH handling", () => {
     }
   });
 });
+
+describe("findPathKey", () => {
+  it("returns PATH when key is uppercase", () => {
+    expect(findPathKey({ PATH: "/usr/bin" })).toBe("PATH");
+  });
+
+  it("returns Path when key is mixed-case (Windows style)", () => {
+    expect(findPathKey({ Path: "C:\\Windows\\System32" })).toBe("Path");
+  });
+
+  it("returns PATH as default when no PATH-like key exists", () => {
+    expect(findPathKey({ HOME: "/home/user" })).toBe("PATH");
+  });
+
+  it("prefers uppercase PATH when both PATH and Path exist", () => {
+    expect(findPathKey({ PATH: "/usr/bin", Path: "C:\\Windows" })).toBe("PATH");
+  });
+});
+
+describe("applyPathPrepend with case-insensitive PATH key", () => {
+  it("prepends to Path key on Windows-style env (no uppercase PATH)", () => {
+    const env: Record<string, string> = { Path: "C:\\Windows\\System32" };
+    applyPathPrepend(env, ["C:\\custom\\bin"]);
+    // Should write back to the same `Path` key, not create a new `PATH`
+    expect(env.Path).toContain("C:\\custom\\bin");
+    expect(env.Path).toContain("C:\\Windows\\System32");
+    expect("PATH" in env).toBe(false);
+  });
+
+  it("preserves all existing entries when prepending via Path key", () => {
+    // Use platform-appropriate paths and delimiters
+    const delim = path.delimiter;
+    const existing = isWin
+      ? ["C:\\Windows\\System32", "C:\\Windows", "C:\\Program Files\\nodejs"]
+      : ["/usr/bin", "/usr/local/bin", "/opt/node/bin"];
+    const prepend = isWin ? ["C:\\custom\\bin"] : ["/custom/bin"];
+    const existingPath = existing.join(delim);
+    const env: Record<string, string> = { Path: existingPath };
+    applyPathPrepend(env, prepend);
+    const parts = env.Path.split(delim);
+    expect(parts[0]).toBe(prepend[0]);
+    for (const entry of existing) {
+      expect(parts).toContain(entry);
+    }
+  });
+
+  it("respects requireExisting option with Path key", () => {
+    const env: Record<string, string> = { HOME: "/home/user" };
+    applyPathPrepend(env, ["C:\\custom\\bin"], { requireExisting: true });
+    // No Path/PATH key exists, so nothing should be written
+    expect("PATH" in env).toBe(false);
+    expect("Path" in env).toBe(false);
+  });
+});