merge function does not correctly merge objects

Environment

const target = {
      sessionToken: {
        name: "authjs.session-token",
        options: {
          httpOnly: true,
          sameSite: "lax",
          path: "/",
          secure: false,
        },
      }
}

const source = {
      sessionToken: {
        options: {
          expires: new Date("2024-01-01T00:00:00Z"),
        },
      }
}

// Call
merge(target, source)

const expected = {
      sessionToken: {
        name: "authjs.session-token",
        options: {
          httpOnly: true,
          sameSite: "lax",
          path: "/",
          secure: false,
          expires: source.sessionToken.options.expires,
        },
      }
}

// Actual result
 Object {
    "sessionToken": Object {
      "name": "authjs.session-token",
      "options": Object {
-       "expires": 2024-01-01T00:00:00.000Z,
+       "expires": Object {},
        "httpOnly": true,
        "path": "/",
        "sameSite": "lax",
        "secure": false,
      },
    },
  }

See PR: https://github.com/nextauthjs/next-auth/pull/13030

Reproduction URL

https://github.com/magom001/operaclass.net/tree/nextauth

Describe the issue

Merge function does not correctly merge objects. E.g., it treats Date as a regular Object.

How to reproduce

Add the following test to merge.test.ts:

it("should handle merging Date object", () => {
    const target = {
      sessionToken: {
        name: "authjs.session-token",
        options: {
          httpOnly: true,
          sameSite: "lax",
          path: "/",
          secure: false,
        },
      },
    }
    const source = {
      sessionToken: {
        options: {
          expires: new Date("2024-01-01T00:00:00Z"),
        },
      },
    }

    const expected = {
      sessionToken: {
        name: "authjs.session-token",
        options: {
          httpOnly: true,
          sameSite: "lax",
          path: "/",
          secure: false,
          expires: source.sessionToken.options.expires,
        },
      },
    }

    expect(merge(target, source)).toEqual(expected)
  })

Expected behavior

merge correctly merges objects

Start with the merge implementation and the existing merge.test.ts file, then add the provided Date-object reproduction. Run the merge tests and confirm that the nested expires value remains a Date while the other sessionToken.options fields are preserved.