Fix StorageException for users with multi-path offline content

[ziad-halabi9]

Summary

• OfflineDataSourceFactoryHelper.getExternal/getInternal cached one factory per storage type and ignored the path argument after the first call. Because the helper is @Reusable in Dagger, that stale factory persisted for the whole app session, so users whose offline content lived at multiple paths within one storage class (e.g. they switched download location, hit an OS-level path migration, or use a different SD-card root) hit StorageException / ERROR_CODE_IO_UNSPECIFIED on later tracks at any path other than the first-seen one.
• Switch the per-storage-type field to a path-keyed Map<String, T> so each distinct path memoizes its own factory. Replace !! with checkNotNull on the nullable provider.
• Add OfflineDataSourceFactoryHelperTest (5 cases): per-path memoization for both getExternal and getInternal, independence of internal/external maps, and IllegalStateException when the provider is null.

Test plan

• ./gradlew :player:playback-engine:testDebugUnitTest (full module suite passes locally on JBR 21)
• New OfflineDataSourceFactoryHelperTest — all 5 cases pass

🤖 Generated with Claude Code

[michpohl]

nit: I'd love if we could use more descriptive naming and given/when/then in the tests, to make them more clear, something like:

@Test
fun `getExternal returns same instance when called multiple times with same path`() {
    // given
    val cacheA = mock<Cache>()
    whenever(offlineCacheProvider.getExternal("a")).thenReturn(cacheA)
    
    // when
    val firstA = helper.getExternal("a")
    val secondA = helper.getExternal("a")
    
    // then
    assertThat(secondA).isSameInstanceAs(firstA)
    ...
}

[michpohl]

Approving with 1 comment. I leave it up to you if you want to change or not.