fix: Include the last data source error in provider error and stale events

[kinyoklion]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

OFP requirements 4.2.3 / 4.2.4 (sdk-specs OFP-openfeature-provider-behavior): STALE/ERROR events MUST include the message of the last data source error, or a message describing the condition if it has none. The Java provider already does this.

Describe the solution you've provided

private static string DescribeLastError(DataSourceStatus status, string fallback)
    => status.LastError is ErrorInfo e ? e.Message ?? e.ToString() : fallback;

• Off (both in StatusChangeHandler and the InitializeAsync pre-check): PROVIDER_ERROR message and the LaunchDarklyProviderInitException message now use DescribeLastError(status, ProviderShutdownMessage) instead of always ProviderShutdownMessage.
• Interrupted: previously LastError?.Message ?? "encountered an unknown error", so HTTP errors (which have no Message) got the generic text; they now get ErrorInfo.ToString() (e.g. kind + status code).

Behavior is unchanged when there is no LastError. Added tests for Off with an exception message, Off with an HTTP 401, Off with no error, and Interrupted with an HTTP 503.

Describe alternatives you've considered

Always using ErrorInfo.ToString() would also include the timestamp for exception-based errors; Message is kept when present so existing Stale messages do not change.

Additional context

Validated locally with dotnet test on net8.0 (72 passed). Independent of the error-type PR (#73).

Link to Devin session: https://app.devin.ai/sessions/929f5530882d40bc95e55391e97536d5
Open in Devin Desktop: https://app.devin.ai/desktop/session/929f5530882d40bc95e55391e97536d5?variant=devin
Requested by: @kinyoklion

SDK-3255

[devin-ai-integration]

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