fix(toolinstall): soft-fail auto-detect installs

[pandego]

Fixes #2384

Summary

• stop treating auto-detected tool install failures as fatal during tool loading
• keep explicit version: installs strict, so pinned package/version mistakes still surface immediately
• add focused regression coverage for the new fallback behavior

Testing

• docker run --rm -v "$PWD":/src -w /src golang:1.26 sh -lc '/usr/local/go/bin/gofmt -w pkg/toolinstall/resolver.go pkg/toolinstall/resolver_test.go && /usr/local/go/bin/go test ./pkg/toolinstall -run "TestEnsureCommand_(AutoDetectFailureFallsBackToOriginalCommand|ExplicitVersionFailureStillErrors|DisabledGlobally|DisabledGlobally_CaseInsensitive|DisabledPerToolset|FoundInBinDir|FoundInPath|NonExecutableInBinDirFallsBackToOriginalCommand)|TestResolve_(FoundInPath|FoundInBinDir|NotFoundAnywhere|InvalidAquaRef)|TestParseAquaRef|TestExtractVersionPrefix" -count=1'