Implement InitializableConfigValueProvider on Firebase provider

[kirich1409]

Summary

Implements InitializableConfigValueProvider on FirebaseConfigValueProvider (closes #163). This was deferred from v1.0.0 pending the API-design decision on initialize() semantics.

initialize() calls FirebaseRemoteConfig.ensureInitialized().await(), which warms Firebase's on-disk caches (activated / fetched / defaults) into memory without a network fetch. After it, ConfigValues.get() reads the persisted values synchronously from memory (Source.REMOTE), enabling instant flag values at app start before the first fetch().

Design decision

ensureInitialized() only — confirmed with the maintainer. Rejected + activate(): activate() would auto-promote config that was deliberately fetched-but-not-activated via fetch(activate = false), changing initialize()'s semantics from "warm the cache" to "warm + auto-activate" — a footgun. ensureInitialized() is the literal match of the interface contract ("load previously cached values, MUST NOT fetch") and does not mutate state.

Error handling mirrors the existing fetch(): CancellationException is re-thrown unwrapped; other exceptions are wrapped in FetchException with the original cause preserved.

Tests

4 unit tests (mockk + runTest):

• ensureInitialized() called exactly once
• fetch() / fetchAndActivate() NOT called (no network)
• task failure wrapped in FetchException with cause preserved
• CancellationException re-thrown without wrapping

:providers:firebase:testDebugUnitTest — 26/26 green; compileDebugKotlin green; spotless clean.

Note: the firebase module's kover threshold is intentionally 0 (#45) — runtime verification against a real Firebase backend is out of scope; the contract is covered by mockk unit tests.

Follow-up (out of scope)

FetchException is fetch-oriented in name but reused for initialize() errors. Consider renaming to a neutral ProviderException in a separate PR.

🤖 Generated with Claude Code

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