feat: improved reliability for failing fetches through forc-pkg

[kayagokalp]

Description

closes #7376 and TOOL-681

Problem: When registry package fetching fails, the created directory remains in ~/.forc/registry/, causing subsequent builds to fail because forc assumes the directory contains a valid package.

To mitigate this:

• Moved directory creation: Directory creation now happens after validating the package exists in the registry index.
• Added cleanup guard: Used scopeguard::guard to ensure automatic cleanup if fetch fails.
• Implemented proper error handling: On successful fetch, the guard is defused; on failure, the directory is automatically removed.

Testing

• test_fetch_directory_cleanup_on_failure validates that failed fetches properly clean up directories

[codspeed-hq]

CodSpeed Performance Report

Merging #7377 will not alter performance

_{Comparing kayagokalp/tool-681 (6de83c0) with master (6981d3d)}

Summary

✅ 25 untouched benchmarks