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When a model called a tool that doesn't exist, the switch statement in
presentAssistantMessage had no default case to handle it. For native
protocol, this meant no tool_result was sent back to the API, causing
the extension to freeze waiting indefinitely.

Added:
- Default case in tool switch statement that:
  - Pushes an error tool_result back to the API
  - Increments consecutiveMistakeCount
  - Records the tool error
  - Shows error message to user
- Comprehensive test suite for unknown tool handling

When using native tool protocol, if a model attempts to call a tool that doesn't exist (like 'edit_file'), the extension would freeze in a loop instead of returning an error to the model.

Root causes:
1. During streaming, partial blocks of unknown tools would reach the default case in the switch statement, showing an error on every streaming chunk
2. validateToolUse was being called for partial blocks, causing repeated validation errors

Changes:
- Add isValidToolName() function to validateToolUse.ts to explicitly check for unknown tools
- Only run tool validation for complete (non-partial) blocks in presentAssistantMessage
- Skip partial blocks in the default case to prevent error loops during streaming
- Fetch state early in tool_use case so mode/customModes are available throughout

This ensures that when an invalid tool is called, the extension waits for the complete tool call before showing a single error and sending a tool_result back to the model.