fix(install): remove overly strict Windows bin linking assertion

Fixes #23414

The code was asserting that all relative paths from .bin to package
binaries must start with "..\\" on Windows. This assumption breaks in
edge cases where the relative path has a different structure.

This commit:
- Replaces the assertion with a conditional check
- Handles both paths that start with "..\\" and those that don't
- Adds a regression test for issue #23414

The fix gracefully handles edge cases while maintaining the expected
behavior for the common case where binaries are in subdirectories
of node_modules.

Co-Authored-By: Claude <noreply@anthropic.com>

test/regression/issue/23414.test.ts

@@ -0,0 +1,44 @@
+// https://github.com/oven-sh/bun/issues/23414
+// Bun crashes when trying to install packages with bin links on Windows
+// This was caused by an overly strict assertion that expected all relative paths
+// to start with "..\\" when linking bins, but edge cases exist where this isn't true.
+
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, isWindows, tempDir } from "harness";
+
+test("bin linking should not crash with assertion failure on Windows", async () => {
+  if (!isWindows) {
+    // This test is Windows-specific, skip on other platforms
+    return;
+  }
+
+  using dir = tempDir("bin-linking-test", {
+    "package.json": JSON.stringify({
+      name: "test-package",
+      version: "1.0.0",
+      dependencies: {
+        "cowsay": "1.6.0",
+      },
+    }),
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "install"],
+    env: bunEnv,
+    cwd: String(dir),
+    stderr: "pipe",
+    stdout: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // The main fix is that this should not panic with:
+  // "panic: Internal assertion failure at install/bin.zig:723:83"
+  expect(stderr).not.toContain("panic");
+  expect(stderr).not.toContain("assertion failure");
+  expect(stderr).not.toContain("bin.zig:723");
+
+  // Verify the install completed successfully
+  expect(exitCode).toBe(0);
+  expect(stderr).toContain("package installed");
+});