Rust local provider should bound and recover from initial CDN state fetch failures

Problem

The Rust local provider exposes ProviderOptions::initialize_timeout, but the option is not applied. Its default reqwest client has no total, connect, or read timeout configured. The initial CDN fetch can therefore wait indefinitely. If it returns an error, initialization sets the provider to Error before background polling is started, so recovery requires recreating or reinitializing the provider.

The README also describes a 30 second default initialization timeout that the implementation does not provide.

Intended behavior

Match the Java local provider startup behavior:

• Apply a bounded initialization/state-fetch timeout, defaulting to 30 seconds, and honor initialize_timeout.
• Initialization should leave the provider NotReady, rather than permanently Error, when initial CDN state is unavailable.
• Flag evaluations should return the caller supplied default with ProviderNotReady.
• Start a background retry loop even when the initial fetch fails.
• Retry every 1 second while NotReady.
• After a successful fetch, transition to Ready and use the configured normal polling interval.
• Shutdown must cancel in-flight fetches and retry tasks.
• Once initialized, preserve the last good state during later CDN failures.

Acceptance criteria

• The configured initialization timeout is actually enforced.
• Startup cannot hang indefinitely on the CDN request.
• Recovery happens automatically without recreating the provider.
• Tests cover timeout, repeated retry, recovery to Ready, default-value evaluation while NotReady, and shutdown cancellation.
• README examples use the actual Duration API and accurately describe timeout behavior.

Start with the Rust local provider implementation and trace ProviderOptions::initialize_timeout through the initial CDN state fetch, provider state transitions, polling, evaluation, and shutdown. Use the acceptance criteria to define tests for timeout, retries, recovery, default-value evaluation, and cancellation. Then update the README examples and timeout description to match the actual Duration API and behavior.