refactor(acp): improve AcpBackend architecture and reliability

- Extract session update handlers into separate module (sessionUpdateHandlers.ts)
  - handleAgentMessageChunk, handleToolCallUpdate, handleToolCall, etc.
  - Reduces AcpBackend.ts from ~1350 to ~900 lines

- Add retry logic for ACP init/newSession operations
  - 3 attempts with exponential backoff (1s, 2s, 4s)
  - Improves reliability on slow starts

- Move timeouts to configurable constants
  - Add getIdleTimeout() to TransportHandler interface
  - Export GEMINI_TIMEOUTS from GeminiTransport
  - DEFAULT_IDLE_TIMEOUT_MS, DEFAULT_TOOL_CALL_TIMEOUT_MS

- Improve tool name detection in GeminiTransport
  - ExtendedToolPattern with inputFields and emptyInputDefault
  - Remove fragile context-based heuristics
  - Cleaner, more declarative pattern matching

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>