⚡ Bolt: Optimize SSE string concatenation in translator packages

[rschumann]

💡 What: Replaced fmt.Sprintf with direct string concatenation (+) in SSE hot paths (e.g., emitEvent, emitRespEvent, and data: block builders) across multiple translator packages (claude, openai, gemini, codex, antigravity). Unused fmt imports were also removed.

🎯 Why: fmt.Sprintf introduces reflection overhead and unnecessary allocations for simple string formatting. This slows down streaming where thousands of chunks are processed per second.

📊 Impact: Reduces formatting time significantly (from ~338ns to ~133ns per operation based on local microbenchmarks) and drops memory allocations per operation from 3 allocs to 1 alloc.

🔬 Measurement: Verified via go test -bench=. -benchmem showing a >60% time reduction and fewer allocations per call compared to fmt.Sprintf. Passed all translator package tests and global short tests.

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PR created automatically by Jules for task 18304292734356149254 started by @rschumann

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