Add filedb feature flag, cleanup code and add tests

[satyakigh]

Summary

Introduces a FileDb feature flag to control the persistence backend. When enabled, the language server uses FileStoreFactory instead of LMDBStoreFactory. This enables a gradual migration away from LMDB via fleet percentage rollout.
The DataStore is selected based on the feature flag and operating system ONCE on startup - we can't hot swap the database even if the feature flag flips.

Key Changes

Area	Change
Feature flag	Added FileDb to alpha (on, 100%), beta (off, 100%), prod (off, 0%)
DataStore	MultiDataStoreFactoryProvider now takes a FeatureFlag and uses it to pick FileStoreFactory vs LMDBStoreFactory
Ownership move	FeatureFlagProvider moved from CfnExternal → CfnInfraCore (core now depends on it for datastore init)
Type safety	FeatureFlagSupplier loops use as const satisfies for proper typed keys instead of Record<string, ...>
Extracted helpers	featureFlagLocalFile() and relationshipLocalFile() — testable path builders
Minor	readBufferIfExists return type NonSharedBuffer → Buffer; sleep exported from Retry.ts
Tests	New tests for AndFeatureFlag, LocalHostTargetedFeatureFlag, buildLocalHost, featureFlagLocalFile, File utils, sleep

Feature Flag settings until we test

alpha  → enabled: true,  fleetPercentage: 100  (fully on)
beta   → enabled: false, fleetPercentage: 100  (ready to flip)
prod   → enabled: false, fleetPercentage: 0    (completely off)

The base branch was changed.