Fix version prefix detection for libraries without v-prefix

[partouf]

Summary

• Fixed incorrect target_prefix: v generation for libraries that don't use 'v' prefixes in their tags
• Preserved user-friendly auto-detection when users type version numbers without prefixes

Problem

The ce-library-wizard was incorrectly generating target_prefix: v for libraries like FusedKernelLibrary that use tags like Beta-0.1.9 without any 'v' prefix. This was reported in compiler-explorer/infra#1755

Root Cause

The check_github_release_exists function was checking both with and without 'v' prefix when validating a specific tag. When checking if vBeta-0.1.9 exists, it would also check Beta-0.1.9, find it exists, and incorrectly return true.

Solution

• Modified check_github_release_exists to only check the exact version specified
• Added check_git_tag_with_fallback function for cases where we want user-friendly fallback behavior
• The smart auto-detection still works: users can type 1.0.0 and it will correctly detect if the repo uses v1.0.0

Test Results

# FusedKernelLibrary (no v prefix)
determine_version_format('Beta-0.1.9') → prefix=None ✓

# nlohmann/json (uses v prefix) 
determine_version_format('3.11.2') → prefix='v' ✓
determine_version_format('v3.11.2') → prefix='v' ✓

🤖 Generated with Claude Code