refactor(gemini): Improve ACP architecture, session stability and history deduplication

[Scoteezy]

Summary

This PR improves the Gemini/ACP backend architecture with better code organization, session stability fixes, and conversation history deduplication.

Changes

ACP Architecture Refactoring

• Extract session update handlers into separate module (sessionUpdateHandlers.ts)
• Reduces AcpBackend.ts from ~1350 to ~900 lines
• Add retry logic for ACP init/newSession (3 attempts with exponential backoff)
• Move timeouts to configurable constants (GEMINI_TIMEOUTS)

Session Stability

• Add session swap synchronization during message processing
• Queue pending swaps and apply between processing cycles
• Prevents race conditions when switching devices rapidly

Model Resolution

• Refactor createGeminiBackend to return GeminiBackendResult
• Single source of truth for resolved model and source
• Remove duplicate model resolution logic from runGemini.ts

History Deduplication

• Add duplicate detection in ConversationHistory
• Skip identical user/assistant messages
• Prevents history inflation from retries or reconnects

Tool Detection Improvements

• Rewrite tool name detection in GeminiTransport
• Add ExtendedToolPattern with inputFields and emptyInputDefault
• Add logging for unknown tool patterns