Unleash / unleash / 29733591597

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feat: await for feature resolver readiness

## What

Adds an explicit readiness lifecycle to the feature flag resolver and
waits for it during startup before the server begins using resolved
flags.

## Why

Some startup paths evaluate feature flags very early, before all
resolver implementations have necessarily finished initializing. Making
readiness part of the resolver contract lets startup coordinate that
dependency directly instead of relying on individual call sites to know
whether a resolver is ready.

This keeps flag evaluation synchronous for callers while making startup
ordering explicit. The startup logs also report how long the resolver
readiness wait took, which makes it easier to identify whether startup
is delayed by resolver initialization.

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