Add 111 unit tests for sync layer and fix HostInfoFilter DNS resolution

[guiyanakuang]

Closes #3815

Summary

• Add 111 unit tests across 6 test classes covering the core sync/discovery/connection layer
• Extract TokenCacheApi interface from TokenCache object for clean dependency injection in tests
• Fix HostInfoFilter.resolveIpBytes() to reject non-IP strings that trigger unintended DNS resolution

Test classes

Class	Tests	Coverage
SyncResolverTest	50	Event dispatch, state transitions, heartbeat, token cache, trust
GeneralSyncHandlerTest	20	State machine, API delegation, address resolution
SyncPasteTaskExecutorTest	13	Eligibility filtering, sync execution, retry logic
TelnetHelperTest	11	Single host telnet, parallel switchHost racing
GeneralNearbyDeviceManagerTest	10	Device add/remove, self-filtering, blacklist
SyncPollingManagerTest	7	Exponential backoff, reset, cancellation

Production changes

• TokenCache.kt: Extract TokenCacheApi interface; TokenCache object implements it
• SyncResolver.kt: Accept TokenCacheApi instead of TokenCache concrete type
• DesktopModule.kt: Bind TokenCacheApi in Koin
• HostInfoFilter.kt: Add looksLikeIpLiteral() guard to prevent DNS resolution of non-IP strings

Test plan

• All 111 new tests pass
• All pre-existing tests pass (HostInfoFilterTest now also passes)
• ktlintFormat passes

🤖 Generated with Claude Code