fix(shell): handle ".", "", "./" in cwd() by using process.cwd() (#26461)

## Summary
- Fix `$`...`.cwd(".")` causing ENOENT error with path ending in
"undefined"
- The same fix applies to `.cwd("")` and `.cwd("./")`
- Falls back to `process.cwd()` when `defaultCwd` is undefined

Closes #26460

## Test plan
- [x] Added regression test in `test/regression/issue/26460.test.ts`
- [x] Verified test fails with `USE_SYSTEM_BUN=1` (reproduces the bug)
- [x] Verified test passes with `bun bd test` (fix works)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

test/regression/issue/26460.test.ts

@@ -0,0 +1,103 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+// Regression test for https://github.com/oven-sh/bun/issues/26460
+// Using $`...`.cwd(".") should work and use the current working directory
+
+test("shell cwd('.') should use current working directory", async () => {
+  using dir = tempDir("shell-cwd-dot", {
+    "test.js": `
+      import { $ } from "bun";
+      const result = await $\`pwd\`.cwd(".").text();
+      console.log(result.trim());
+    `,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "test.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout.trim()).toBe(String(dir));
+  expect(exitCode).toBe(0);
+});
+
+test("shell cwd('') should use current working directory", async () => {
+  using dir = tempDir("shell-cwd-empty", {
+    "test.js": `
+      import { $ } from "bun";
+      const result = await $\`pwd\`.cwd("").text();
+      console.log(result.trim());
+    `,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "test.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout.trim()).toBe(String(dir));
+  expect(exitCode).toBe(0);
+});
+
+test("shell cwd('./') should use current working directory", async () => {
+  using dir = tempDir("shell-cwd-dotslash", {
+    "test.js": `
+      import { $ } from "bun";
+      const result = await $\`pwd\`.cwd("./").text();
+      console.log(result.trim());
+    `,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "test.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout.trim()).toBe(String(dir));
+  expect(exitCode).toBe(0);
+});
+
+test("Shell prototype cwd('.') should use current working directory", async () => {
+  using dir = tempDir("shell-proto-cwd-dot", {
+    "test.js": `
+      import { $ } from "bun";
+      const shell = new $.Shell();
+      shell.cwd(".");
+      const result = await shell\`pwd\`.text();
+      console.log(result.trim());
+    `,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "test.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout.trim()).toBe(String(dir));
+  expect(exitCode).toBe(0);
+});