⚡ Bolt: Remove fmt.Sprintf in hot paths

[rschumann]

💡 What: Replaced fmt.Sprintf with string concatenation (+) and strconv in SSE event building and Data URI construction across multiple translators.
🎯 Why: fmt.Sprintf uses reflection which introduces unnecessary overhead when executing hot paths per chunk in streaming requests.
📊 Impact: Reduces reflection overhead and per-chunk memory allocations, decreasing latency on streaming and vision requests.
🔬 Measurement: Unit tests verified passing via go test ./... -short. Profiling string concatenation vs fmt.Sprintf on hot paths consistently demonstrates lower allocation rates.

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PR created automatically by Jules for task 12705447485444470490 started by @rschumann

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