Support a Client Readiness check in local evaluation mode

从 flagsmith/flagsmith.py 中第 252-259 行附近的远程回退开始，然后跟踪初始化、本地评估模式以及 get_environment_flags/get_identity_flags。定义并测试就绪状态，并确保在本地模式下缺少环境文档时返回默认 flags，而不会在每次评估时调用远程，同时后台轮询器可以恢复。

Problem
Currently, when a Flagsmith client is initialized, it makes a blocking remote call to the Flagsmith endpoint to fetch the environment document. If this fetch fails, the exception is simply logged with an error message.

The issue arises during subsequent flag evaluations (get_environment_flags / get_identity_flags). If the initial fetch failed, the SDK defaults to fetching the flags via a remote API call.

This behavior is highly undesirable when the client is explicitly configured for local evaluation mode.

Systemic Impact
If the client is unable to fetch the environment document because the Flagsmith edge/proxy is experiencing high load or degradation, the default API fallback mechanism drastically worsens the situation. Instead of polling for the document occasionally, the client begins making a remote API call for every single flag evaluation. This creates a thundering herd effect, overwhelming the proxy and potentially causing a death spiral where the proxy can never recover to a healthy state. Also, now clients are no longer evaluating flags locally resulting in latencies in the call path.

Proposed Solution
We need a way to verify the client's state and prevent unwanted remote API spam.

Introduce a ready() API: The SDK should expose a ready() method (or an is_ready property) that developers can use to check for client readiness before proceeding with flag evaluations in the evaluation path.

Re-evaluate the Fallback Behavior: When in local evaluation mode, the SDK should ideally not fall back to remote API calls on a per-evaluation basis if the environment document is missing. Instead, it should return default flags and rely on the background poller to eventually fetch the document once the proxy recovers.

Expected Behavior
In local evaluation mode, a failure to fetch the initial environment document should not silently convert the client into a remote-evaluation client. Developers should be able to check if the local cache is populated and healthy via a ready() method.