feat(detect): add --no-fallback option

[railgun-0402]

Summary

• Add --no-fallback flag to tenv tofu detect and tenv tf detect commands
• When set, return exit code 1 if no version files are found instead of falling back to default strategy

Motivation

Closes #502

Users need to distinguish whether a folder is configured for OpenTofu or Terraform. Currently, tenv tofu detect and tenv tf detect return exit code 0 even when no version files are found, because
a fallback mechanism triggers to select a default version.

Changes

• versionmanager/manager.go: Add ErrNoVersionFilesFound error and ResolveStrict() method
• cmd/tenv/subcmd.go: Add --no-fallback flag to detect command
• versionmanager/proxy/proxy.go: Update Detect() call signature
• versionmanager/tenvlib/lib.go: Update Detect() call signature

Test plan

# In a directory without version files                                                                                                                                                                  
cd /tmp/test-dir                                                                                                                                                                                        
tenv tofu detect --no-fallback                                                                                                                                                                          
echo $?  # Returns 1                                                                                                                                                                                    
                                                                                                                                                                                                        
# With version file                                                                                                                                                                                     
echo "1.6.0" > .opentofu-version                                                                                                                                                                        
tenv tofu detect --no-fallback                                                                                                                                                                          
echo $?  # Returns 0                      

[kvendingoldo]

Hi! Thank you for PR!
Me or @dvaumoron will check it soon