game.objective still overwritten by change_grammar() when quests exist (regression of #239)

Description

Issue #239 reported that manually setting quest.desc or game.objective gets overwritten by the text generation process during build()/compile(). PR #240 added a fix that copies _objective from a previous build before change_grammar() runs.

However, the fix is incomplete. In build(), the copy happens at line 788-789 before change_grammar() is called at line 803. change_grammar() then unconditionally overwrites the objective at line 460 of game.py:

self.objective = describe_event(Event(policy), self, self.grammar)

This means the preserved objective gets clobbered whenever the game has quests with a winning policy (i.e. most games).

The test added in PR #240 (test_manually_defined_objective) only tests a game with no quests, so it doesn't catch this.

Steps to Reproduce

import textworld
from textworld import GameMaker

M = GameMaker()
r1 = M.new_room("bedroom")
M.set_player(r1)

key = M.new(type="k", name="key", desc="This is a skeleton key.")
r1.add(key)

quest = M.set_quest_from_commands(["take key"])

game = M.build()
game.objective = "Find a valuable object."

game_file = M.compile("/tmp/test_game.z8")

# Load and check - the objective has been overwritten
loaded = textworld.Game.load("/tmp/test_game.json")
assert loaded.objective == "Find a valuable object.", f"Expected custom objective, got: {loaded.objective[:80]}..."

The assertion fails because change_grammar() in the second build() (triggered by compile()) overwrites the objective with auto-generated text.

Proposed Fix

In game.py, change_grammar() should only set the objective if one hasn't already been set:

# Before (line 460):
self.objective = describe_event(Event(policy), self, self.grammar)

# After:
if self._objective is None:
    self.objective = describe_event(Event(policy), self, self.grammar)

Workaround

Until this is fixed, you can work around it by calling M.build(), setting game.objective, then compiling manually with textworld.generator.compile_game(game, options) instead of M.compile().

Environment

• TextWorld 1.7.0
• Python 3.12
• macOS

Start in game.py at change_grammar() and inspect the objective preservation logic in build(); review the existing test_manually_defined_objective case. Add coverage for a quest with a winning policy and verify that a manually set objective remains unchanged after compile and load.