fix: move now = datetime.now() inside the lock to avoid stale timestamp

[yunyancuo]

问题

当多个协程等待同一个会话锁时，它们都在进入锁之前就捕获了 now = datetime.now()。等到协程真正获取到锁时，捕获的时间戳已经过时了——它反映的是等待前的时间，而不是实际获取锁的时间。这会导致过期时间戳未被清理，限流窗口从错误的时间点计算。

修复

将 now = datetime.now() 移到 async with self.locks[session_id]: 块内部，确保每个协程在获取锁的那一刻才获取当前时间。

测试

在 tests/test_rate_limit_stage.py 中新增 test_stalled_concurrent_events_use_current_time_after_lock：

• 使用虚拟时钟模拟并发 stall 事件
• 验证竞争条件下的 sleep 持续时间正确
• 验证时间戳单调有序

Fixes #9348

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[sourcery-ai]

Hey - I've left some high level feedback:

• The test monkeypatches rate_limit_stage.datetime with FakeDateTime, which only implements now(); if the module ever uses other datetime attributes this test will start failing in non-obvious ways—consider monkeypatching just datetime.now instead of the entire symbol.
• The expected sleep_durations == pytest.approx([60.3, 60.3]) is a bit opaque; consider deriving these values from rate_limit_count and rate_limit_time (or adding a short comment) so the relationship between configuration and expected sleeps remains clear if the defaults are adjusted.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The test monkeypatches `rate_limit_stage.datetime` with `FakeDateTime`, which only implements `now()`; if the module ever uses other `datetime` attributes this test will start failing in non-obvious ways—consider monkeypatching just `datetime.now` instead of the entire symbol.
- The expected `sleep_durations == pytest.approx([60.3, 60.3])` is a bit opaque; consider deriving these values from `rate_limit_count` and `rate_limit_time` (or adding a short comment) so the relationship between configuration and expected sleeps remains clear if the defaults are adjusted.

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[yunyancuo]

感谢 review！已根据反馈修改：

1. 仅 mock datetime.now 而非整个 datetime 符号：由于 datetime.datetime 是 C 扩展类型，无法直接修改其 now 属性。我将 FakeDateTime 改为继承 datetime.datetime 的子类，只重写 now()，其他所有 datetime 属性和方法都通过继承保留——如果模块后续用到其他 datetime 功能，也不会出问题。

2. 期望值从配置推导：将硬编码的 pytest.approx([60.3, 60.3]) 改为 expected_stall = limiter.rate_limit_time.total_seconds() + 0.3，明确表达期望 sleep 时长与限流配置（rate_limit_time=60，margin=0.3）之间的关系。