⚡ Bolt: Replace fmt.Sprintf with direct string concatenation in SSE streaming paths

[rschumann]

💡 What: Replaced fmt.Sprintf("data: %s\n\n", ...) and data URL formatting with direct string concatenation and strconv.FormatInt across all translator streaming handlers.
🎯 Why: fmt.Sprintf incurs reflection overhead and unnecessary allocations per chunk. In streaming handlers which are hot paths, direct string concatenation avoids this reflection penalty and improves performance.
📊 Impact: Reduces allocations and improves throughput. Benchmarks show a ~30% latency reduction (199ns to 137ns) in standard SSE chunk formatting.
🔬 Measurement: Microbenchmark tested locally, verified with standard test suite.

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PR created automatically by Jules for task 11902130841854047432 started by @rschumann

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