fix: allow lazy load evaluations when $inited key is not set

[kinyoklion]

Summary

In lazy load / daemon mode, ClientImpl::PreEvaluationChecks gates all flag evaluations on Initialized(). For LazyLoad, Initialized() checks whether the $inited key exists in the persistent store (e.g. Redis). In daemon mode, an external process (Relay Proxy, another SDK) populates the store, but the $inited key may not always be present — causing every evaluation to return CLIENT_NOT_READY even though flag data is available for on-demand fetching.

This PR introduces a CanEvaluateWhenNotInitialized() virtual method on IDataSystem (defaults to false). LazyLoad overrides it to return true, since it can always fetch data on demand. The evaluation path (PreEvaluationChecks and AllFlagsState) now checks this method: when the data system reports it can evaluate while not initialized, a warning is logged and evaluation proceeds instead of blocking.

LazyLoad::Initialized() continues to truthfully report whether $inited is set — consistent with other SDK implementations. The behavioral change is in the evaluation path, not the initialization reporting.

Files changed:

• idata_system.hpp — adds CanEvaluateWhenNotInitialized() virtual method (default false)
• lazy_load_system.hpp — overrides CanEvaluateWhenNotInitialized() to return true
• client_impl.cpp — PreEvaluationChecks and AllFlagsState warn-and-proceed when data system can evaluate while not initialized
• lazy_load_system_test.cpp — adds test for CanEvaluateWhenNotInitialized()

Review & Testing Checklist for Human

• Warning log volume: PreEvaluationChecks logs a warning on every evaluation call when $inited is missing. There is no rate-limiting or "log once" guard. In a busy daemon-mode deployment without $inited, this could produce extremely noisy logs. Consider whether a once-per-session or rate-limited warning is more appropriate.
• StartAsync() behavior: LazyLoad::Initialize() only sets kValid if Initialized() returns true. When $inited is absent, status stays at kInitializing and the StartAsync() future may never resolve. Verify this is acceptable — evaluations proceed (via the new warning path), but the client may never report as fully "started."
• AllFlagsState when store is empty/unreachable: Previously returned {} immediately when not initialized. Now for LazyLoad it proceeds to call AllFlags() / AllSegments() on the store. Verify behavior is reasonable if the store is genuinely empty vs. just missing $inited.
• Contract test verification: This fix addresses Issue feat: eventsource client #1 from PR chore: Add support for persistent store contract tests. #502's persistence test failures. After both PRs are merged, run the persistent data store contract tests end-to-end against a Redis instance to verify daemon-mode tests pass.

Notes

• This was not built or tested locally (CMake toolchain was not available on the dev machine). CI is the first compilation check.
• The Erlang SDK uses a similar pattern: it distinguishes not_initialized (blocks) vs store_initialized (warns but proceeds).
• The Go SDK bypasses the initialization check entirely in daemon mode by using a NullDataSource that always reports initialized.
• Related: PR #502 (persistence contract tests) — that PR has a separate TTL field name fix; this PR addresses the Initialized() gating issue.

Link to Devin session | Requested by: @kinyoklion

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