fix: bootstrapped client can do evaluation when not ready

[joker23]

This commit will allow variation calls to use bootstrapped values during the client initialization process.

The bootstrap configuration will only be done once per client instance as we expect this fall back will only be active for a brief amount of time.

---

Note

Allow bootstrapped flag values to be used during initialization by reading bootstrap once in the browser client and seeding flags via a new protected setBootstrap in the shared client.

• Browser (packages/sdk/browser/src/BrowserClient.ts):
  • Read bootstrap data in identifyResult (only once per instance) via readFlagsFromBootstrap and seed flags with setBootstrap before async identify completes; log on failure.
  • Add _bootstrapAttempted guard to prevent repeated bootstrapping.
• Shared (packages/shared/sdk-client/src/LDClientImpl.ts):
  • Add protected setBootstrap(pristineContext, newFlags) to set context and seed flags in FlagManager.
  • Import ItemDescriptor for bootstrap flag descriptors.

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