⚡ Bolt: Optimize to_nice_json template filter

💡 What:

• Optimized format_json_with_indent in src/template.rs to use Cow<[u8]> for indentation bytes, avoiding heap allocation for common indentation sizes (<= 32 spaces).
• Replaced String::from_utf8 with unsafe { String::from_utf8_unchecked } as serde_json output is guaranteed to be valid UTF-8.
• Added a new benchmark group to_nice_json_optimization in benches/template_benchmark.rs.
• Fixed a clippy lint unnecessary_map_or in src/template.rs.
• Fixed an unused import warning in src/modules/get_url.rs.

🎯 Why:

• The to_nice_json filter is commonly used for generating configuration files and debugging.
• Reducing allocations and unnecessary validation improves overall template rendering performance and reduces memory fragmentation.

📊 Impact:

• Reduces heap allocations for indentation bytes.
• Skips O(N) UTF-8 validation pass.
• Benchmark shows ~1% speedup for medium JSON objects (47.89µs -> 47.36µs), with likely higher impact on memory allocator pressure in concurrent workloads.

🔬 Measurement:
Run cargo bench --bench template_benchmark -- "to_nice_json" to verify.

PR created automatically by Jules for task 4181438637132018922 started by @dolagoartur