fix(v3): dirty union bug

[mmkal]

bug repro: https://stackblitz.com/edit/stackblitz-starters-swdj3e?file=index.test.js&startScript=test

Basically, zod v3 wrongly reports a "normal" error rather than a union error for a union of strict objects (and presumably other unions that set status=dirty:

import { test, expect } from 'vitest';
import { z } from 'zod';

test('hmm', () => {
  const union = z.union([
    z.object({ x: z.number() }).strict(),
    z.object({ y: z.number() }).strict(),
  ]);

  expect(union.safeParse({ x: 1, y: 1 })).toMatchInlineSnapshot(`
    {
      "error": [ZodError: [
      {
        "code": "unrecognized_keys",
        "keys": [
          "y"
        ],
        "path": [],
        "message": "Unrecognized key(s) in object: 'y'"
      }
    ]],
      "success": false,
    }
  `);
  expect(union.safeParse({ x: 1, y: 1 })).toMatchObject({
    success: false,
    error: {
      issues: [
        {
          code: 'invalid_union',
          unionErrors: [expect.any(z.ZodError), expect.any(z.ZodError)],
        },
      ],
    },
  });
});

Note: this does not repro in v4. I'm not sure if there's anything going into zod v3 still but opening this against main in hopes that it can get fixed in v3.

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