⚡ Optimize file search with concurrent fetching

[myaple]

💡 What:

• Replaced sequential get_file_content calls in FileIndexManager::search_files with concurrent execution using futures::future::join_all.
• Added a new benchmark test src/tests/file_indexer_perf_test.rs using wiremock to simulate network latency and verify the performance improvement.

🎯 Why:

• The previous implementation fetched file contents one by one. For a search result of 5 files (the limit), this meant the user had to wait for 5 sequential round-trips to the GitLab API.
• This optimization allows all 5 requests to happen in parallel, reducing the total wait time to approximately the duration of the slowest single request.

📊 Measured Improvement:

• Baseline (Sequential): ~518ms (for 5 files with 100ms simulated latency).
• Optimized (Concurrent): ~110ms (for 5 files with 100ms simulated latency).
• Speedup: ~4.7x faster.

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PR created automatically by Jules for task 10613311397338644940 started by @myaple

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