⚡ Pre-allocate slice capacity for user prompt parts

[Gerifield]

💡 What: Modified internal/ai/gemini/logic.go to pre-allocate the capacity of the userPromptParts slice using make([]*ai.Part, 0, len(req.InlineData)+1).

🎯 Why: To prevent multiple underlying array re-allocations as req.InlineData and the final req.Message are appended to the slice, improving memory efficiency.

📊 Measured Improvement: Direct measurement with go test -bench was not possible in this environment due to restricted internet access and timeout issues when fetching dependencies. However, pre-allocating slice capacity is a well-established Go best practice that reduces CPU cycles spent on memory management and copying during slice growth. By setting the capacity to len(req.InlineData) + 1, the slice is guaranteed to fit all elements without resizing.

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PR created automatically by Jules for task 11223308840990628517 started by @Gerifield

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