Model-Aware Image Processing Limits

Image processing now respects per-model limits for dimensions and file size, rather than using global constants:

• maxWidth, maxHeight, maxBase64Size, and targetRawSize can now vary by model
• Default limits remain at 2000×2000 pixels, 5MB base64, and ~3.75MB target raw size
• The imageLimits parameter is threaded through clipboard paste, content processing, and tool result handling
• Image validation now also checks images inside tool_result content blocks (not just top-level user messages)