Implement Phase 3 Claude provider improvements

Add three robustness improvements to ClaudeProviderHandler:

1. Message Role Validation (_validate_messages):
   - Validate roles are one of: user, assistant, system, tool
   - Auto-fix unknown roles to 'user'
   - Ensure system messages only appear at start
   - Insert synthetic assistant messages between consecutive user messages
   - Merge consecutive assistant messages
   - Validate tool messages have tool_call_id
   - Reference: vendors/kilocode normalizeMessages() + ensure_alternating_roles()

2. Tool Result Size Validation (_truncate_tool_result):
   - Truncate oversized tool results with configurable limit (default 100k chars)
   - Add truncation notice with original length info
   - Reference: vendors/claude applyToolResultBudget

3. Model Fallback Support (handle_request refactoring):
   - Add _get_fallback_models() to read fallback list from config
   - Retry with fallback models on retryable errors (rate limit, overloaded)
   - Split into handle_request() (with retry) and _handle_request_with_model() (actual logic)
   - Log fallback attempts for debugging

All methods integrated into handle_request() for automatic application.