fix: Resolve initialization immediately when a start wait time was used

[kinyoklion]

A positive StartWaitTime no longer causes a second wait during provider initialization.

• The provider constructor already blocks in the LdClient constructor for the start wait time, so InitializeAsync now completes with the outcome instead of scheduling its own timer; a 5 second start wait no longer means up to 10 seconds before SetProviderAsync resolves.
• A zero StartWaitTime is unchanged: no timeout, initialization completes when the data source becomes valid or fails permanently.
• The failure message changed to the client did not become ready within the {n}ms start wait time.

Implementation details

Follow-up to #60, which introduced the timeout. ScheduleInitTimeout is replaced by FailInitializationIfNotReady, called at the end of InitializeAsync when a start wait was configured:

if (_startWait.HasValue)
{
    FailInitializationIfNotReady(_startWait.Value);
}

The provider still keeps listening for data source status after failing, so a later successful connection emits a ready event and evaluations are never short-circuited.

Testing: dotnet test test/LaunchDarkly.OpenFeature.ServerProvider.Tests -f net8.0. The timeout test now asserts the immediate failure, and the "does not time out when the client becomes ready" test covers a client which is ready when initialization is requested.

Related spec change: https://github.com/launchdarkly/sdk-specs/pull/257

Link to Devin session: https://app.devin.ai/sessions/0c452d209ec54b068ba120b4c92b8f6c
Open in Devin Desktop: https://app.devin.ai/desktop/session/0c452d209ec54b068ba120b4c92b8f6c?variant=devin
Requested by: @kinyoklion

---

Note

Overview
Fixes double-wait on provider startup when StartWaitTime is greater than zero. The LaunchDarkly client already blocks in the provider constructor for that duration; InitializeAsync no longer schedules a second timer, so a 5s start wait no longer stretches toward ~10s before SetProviderAsync completes.

When a start wait is configured and the client is still not ready after construction, InitializeAsync fails synchronously with an updated message (the client did not become ready within the {n}ms start wait time). Background reconnection is unchanged—a later successful connection can still emit a ready event. StartWaitTime of zero still means non-blocking construction and async initialization until the data source is valid.

README initialization notes are aligned with this behavior; unit tests assert immediate failure and the ready-client success path.

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